Design numbers

[TCLynx]

So how many fish can I have in my 300 gallon tank? or how much Raft do I need to support 60 lb of fish? This is the place to share the numbers for designing a well balanced system.

I know the numbers for Flood and drain media so that is what I will share and I'll let the experts on rafts share that info.

Standard rule for media systems is (Actually this is MAX and only for the skilled fish keeper.)
3 kg of fish per 100 liters of flood and drain media (those fish can be living in 50-100 liters of water, and a sump tank could be necessary to provide extra water if the fish tank is on the small side of the numbers) So to convert that to measure that we think in more commonly.
5 lb of fish per 25 gallons of flood and drain media.

A more appropriate recommendation for fish stocking would be 20-25 fish (500 gram grow out) per 500 liters of grow bed or 1 fish per cubic foot of grow bed with a planned grow out of 1 lb.  Total grow bed and fish tank should be of equal volume or if a sump tank or indexing valve is used there can be more grow beds than fish tank up to twice as much grow bed as fish tank.

Perhaps this might be a little conservative but it is better to start light on fish while cycling up a new system. Once you get comfortable you can decided how best to keep the fish.
A big note here. Those fish weights are for the planned final grown out weight of fish in a system.

Example, say you have a 300 gallon tank and 300 gallons of flood and drain media filled grow bed.
Say it's a simple system pumping from the fish tank to the grow beds which drain back to the fish tank. Say you are growing tilapia and you plan to grow them out to 1 lb if you can.
I would say stock only about 60 fish. (I'm not into planning on lots of fish deaths so I would only personally pad this number by a few fish-I would not personally stock 100 fish and plan on loosing 40.) Again this is assuming that you will grow all those fish out to eating size in that same tank. Again these numbers are a bit skewed, Kinda figuring people would start eating their tilapia a bit smaller than a pound to make enough room for the other fish to get to a pound.  Also, tilapia tend to allow people to pull off higher numbers than are really possible with most other types of fish.  In such a 300 gallon 1:1 system I am currently growing only 30 catfish.

They start small in a new system and give you time for the bio-filter to cycle up to the load.

Some other handy numbers for figuring out pump sizing and aeration.
You want your water pump to cycle the equivalent of the volume of you fish tank each hour at the amount of head you are requiring of it. So for that 300 gallon system, if you run the pump continuously and use auto siphons to drain the grow bed, you need a pump that will do about 400 or 500 gallons an hour. If you wish to use a timer and flood all the beds together 15 minutes per hour, you need a pump that will move that 400-500 gallon in15 minutes so you need a pump that can do 1600-2000 gallons per hour. Another option is an aquaponic indexing valve where you could pump for a period of time to each bed in sequence. Figuring out the pump required for this will depend on the indexing valve chosen and the timer one wishes to use.

On pumps, do some extra research, a cheap pump often uses more electricity so it may cost more in the long run than the costly pump. Always look for performance curves to tell how much flow can be expected at different head heights. Remember that small plumbing will reduce efficiency, never restrict your pump output.

On to air pumps. This will definitely be different than the DWC system since for a media system, you only need to aerate fish tanks/sump tanks. Aeration of tanks helps keep solids moving along and not collecting in with the fish. Aeration will also assist the fish in being able to metabolize food and grow faster. If there is not enough oxygen, fish don't eat as well. Highly aerated water also assists the bacteria where ever they may form to convert ammonia and nitrite to nitrate. It also assists in keeping the water circulated to avoid dead spots and brings bottom water up to the surface for better aeration in that way (it isn't just the bubbles contact with water that aerates.)

Aeration numbers. Simple rules of thumb I've been told.
You want 1 cubic foot per minute of air for each 400 gallons of fish tank. You need 1 psi to push air though the air stone and 1 psi to push air down 28 inches under water. So for the average system you want to figure out how much air an air pump will provide at 2 psi and then figure out how many cubic feet per minute you need to provide for your system volume.
I have a 60 watt air pump that provides 2 CFM at 2 psi. That air pump runs all the time. Normally it runs on mains power but I have installed a relay so that if mains power goes out, I have a deep cycle battery that will keep the air running to my system for at least a day. (So there is aeration and backup all in one air pump plus the appropriate batter, charger, inverter and a DTDP relay.)

Air I'm sure is a completely different story in Raft culture and I've heard of raft systems being run almost completely on air with minimal water pumping. But I know almost nothing about blowers so I'll leave that explanation to others.

Some notes about plumbing. Gravity drains always need far larger plumbing than their pumped counterparts. For example. If you are pumping into a tank using 1" plumbing, you had better make sure your gravity drains and overflows out of that tank are far larger than the 1" inlet or the tank will overflow. Don't make your overflows too close to the rim of the tank or they might not have a chance to work before the tank actually overflows over the top. You need enough fall or height for many gravity drains to work, it might not have to be a lot but you can't drain uphill by gravity. So if you are gravity draining from a fish tank, make sure the place to which the water drains to is lower than the high water level in the fish tank or it won't work.

If in doubt about plumbing, go bigger. It is easy to reduce down later if you need to but if the hole or bulkhead is too small, it can be difficult to up-size later.

[TCLynx]

Sylvia, I would be careful of giving the 5 gallons per pound number since that is really approaching MAX density for a 1:1 system and many people get rather confused by the ratios. I notice around here you seem to lately be using the ratios in the reverse order to how I've been used to seeing them too. Anyway, I don't normally like to use the MAX numbers anymore.

I would be more supportive of using the recommended stocking rates of 1 fish (with 1 lb grow out) per cubic foot of grow bed media or in liters it would be 20-25 fish per 500 liters of grow bed (with 500 gram grow out.) As many of the Aussies have found, if some one needs to ask how many fish they can stock, they probably are not prepared to handle MAX. And if you tell them the MAX numbers, some people see that as the starting point and think they should stock some extra in case some of them die, which is more likely to set them up to kill them all. Grow beds and fish tank volumes Should run about even unless one is running a sump tank or Indexing valve in which case there can be more grow bed than fish tank. The 1:2 ratio of fish tank to grow bed is common, probably because adding more grow beds beyond that doesn't really allow for more fish since you can only keep so many fish in a given volume of water before you start having to pump pure oxygen in to keep the dissolved oxygen levels up.
I also like to focus on the filtration volume a bit more than the fish tank. Seems most people will tend to get a big fish tank and focus on how many fish they can put in it and then add a little bit of grow bed as an afterthought and I would rather focus the other way around. Fit in as much grow beds for the fish you think you want and leave just enough space for the fish tank/sump tank needed for that amount of fish/grow beds. Seems most people are rather shocked at how may plants and gravel beds are needed for a heavy fish load in a given fish tank volume.

If you run huge volumes of fish tank with only a small amount of grow bed, that can be really hard to balance too which is why the 1:1 to 1:2 ratios of fish tank to grow beds are generally the range to be in. If the fish tank is really big and the stocking levels are only at the rate appropriate for the grow beds, well there is still an awful lot of water to try to get filtered in any given period of time, this is difficult with only a small amount of grow bed. Extra water can add stability but only if there is enough flow, aeration and filtration to make sure dust and debris falling into the water or algae growing in the water don't degrade the water quality and that the nutrients can some how be collected and gotten to the plants.

[Sylvia Bernstein]

I think all of that was your way of agreeing with what I said earlier about the ratio of fish tanks to grow beds (except for pointing out my dyslectic error last week).

Now for stocking density. My quest is to get to some Basic rules of thumb without a lot of verbiage around them. My simple mind finds flipping between gallons and cubic feet then over to metric confusing. If you do all the conversions what you are asserting is that stocking ratios should be between 1 fish (or 1 lb of fish for mature bio-filters) per 5 - 7.5 gallons of tank water, with the higher density (5 gallons) being for more experienced gardeners. c'est vrai?

While I accept that 1 fish per 5 gallons is about the max of where you want to be, the world does not necessarily accept our assignment of "max" stocking density. I just heard from a woman I got to know recently through helping her with her aquaponics chapter in her urban homesteading book told me that a local 501c3 using a Growing power style system has stocked their tanks at a 1:1 ratio! They have only had the fish in for 3 months, but I'm afraid they are facing a disaster.

[TCLynx]

I understand the issue with converting and such and I was only focusing on the 1 cubic foot as being an easy number to remember.

If you wanted to recommend 1 fish per 7 gallons of grow bed. (confusion here is that if one does 1 fish per 7 gallons of fish tank and then runs a 1:2 fish tank to grow bed, then one is stocking half the amount one could. Now I don't personally find this a problem since it would probably save a lot of new fish keepers from killing their fish but it will skew the stocking ratios and make some think you can't grow as many fish this way.)

[TCLynx]

Sylvia Bernstein said:

While I accept that 1 fish per 5 gallons is about the max of where you want to be, the world does not necessarily accept our assignment of "max" stocking density. I just heard from a woman I got to know recently through helping her with her aquaponics chapter in her urban homesteading book told me that a local 501c3 using a Growing power style system has stocked their tanks at a 1:1 ratio! They have only had the fish in for 3 months, but I'm afraid they are facing a disaster.

I've experienced (all be it online) a few of these kinds of disasters unfold. Some of them being warned repeatedly even before they get their fish that they needed more filtration and they were given many good recommendations. Then they went on with their original plan and then a few weeks later be on asking for help, how do I get my system cycled NOW I have or it's cycled but I still have ammonia and nitrite readings but then they refuse to believe that they have too many fish for their filtration or that they need supplemental aeration if they have that many fish or whatever. And some times, they pull it off without killing all their fish. I know a skilled fish keeper can manage amazing things if they know what to do and what to watch out for and when water changes can't be avoided.

I know that a commercial system with a focus on the fish can stock far more fish than would ever be appropriate for a backyard system. After all a commercial system will have some one on site doing water tests daily and checking filters and flows and dissolved oxygen and any really big facility is likely to have automated alarms set up to call the owners or managers to come in 24/7 to deal with problems if they arise and many of the places running super high stocking are going to be injecting oxygen rather than just bubbling atmospheric air. What is appropriate for a commercial set up should definitely not sway the recommendations you give for beginning backyard keepers.

[Sylvia Bernstein]

So now let's move on to another topic - how about timing cycles for those of us who aren't using auto siphons. I run my pumps for 15 minutes on, 30 off. Joel at backyard once told me he runs for 15 on, 45 off. Does anyone else have something different?

[Sylvia Bernstein]

So do you have constant flow through your media beds?

[TCLynx]

15/30 or 15/45 or even 15/15 I have done them all. Whatever turns over at least the volume of the fish tank each hour is in my opinion the minimum recommended level which of course has to also be related to pump flow rate at the head and through the plumbing in use. The 15 minutes on time is really just the most easily settable when using the mechanical timers.

People often come on and ask what is the best timing for AP, the truth is, the best timing is the one you can set that will take care of all the other parameters (flow, aeration, water quality/filtration, tank turn over etc.) I've heard of systems that use separate bio-filters and only flood the beds a few times a day and leave them drained most of the time. I've also heard of the reverse where people leave their beds flooded all the time and provided there is enough flow through them, they have done fine too so I don't think there is a hard and fast rule about what is best since such a wide range of things actually seem to work quite well.

Currently my beds that are getting timed flow, here is the timing that is happening at the moment.
300 gallon system
water pumps into the bed for almost 9 minutes and the bed is allowed to drain for a little over 20 minutes. This is happening by indexing valve and repeat cycle timer so the pump is actually on for almost 9 minutes and off for about 100 seconds and it repeats to go to the next bed. Once we add the other three beds to the system each bed will get flow for about 9 minutes out of each hour.

The Big system, the timed beds via indexing valve.
100 gallon beds get flow about 9 minutes 30 seconds out of each hour, there are six beds on that cycle.
Line of barrels Pump on for about 7 1/2 minutes and off for 71/2 minutes making each barrel get flow for about 7 1/2 minutes out of each hour and a half.

These all seem to work. I don't recommend the digital timers as they will not allow hourly cycles 24/7 so unless some one was to run the pump for much longer each hour and only let it be off for brief periods to let the beds drain, I don't think digital timers are appropriate. Mechanical timers should be chosen with care since the cheap ones do fail. I've been having good luck so far with the GE 15 minute increment outdoor mechanical timer.

The other timer option is to get a cycle timer or repeat cycle timer that can be set for a specific on time and off time that will simply repeat as long as there is power. These tend to be a bit costly and this is why most people opt for the mechanical timers instead.

Another important note, it is ok for the pump to keep running for a time even after the water reaches the top of the stand pipe so long as the drain of the grow bed can keep up with the inflow (as in you don't want water flooding over top of your gravel.) This extra water flowing down the top of the stand pipe is getting filtered and providing extra aeration so it is actually a good thing.

[TCLynx]

I know of many who hate siphons who would disagree with you there but myself, if the siphon is working for you, well then it's working and if it ain't broke, don't fix it.

Many people do have trouble getting siphons to work properly though and other than the odd failed timer, the stand pipe and timer method is fairly simple. Main draw back I see is when the pump isn't running, the supplemental aeration becomes more necessary and to turn over the volume of the fish tank in a timed pumping situation, the pump actually needs to be bigger to move more water in less time.

Constant pumping has the benefit of constant flows, aeration, filtration and usually is able to get away with using a smaller pump.

When you start getting into rather large systems, you start to run into the issue that an inexpensive mechanical timer is not going to be rated to handle the load of a large pump turning on/off and heavier duty timers become necessary or relay switches or something like that.


Kobus Jooste said:

I have constant flow to the beds, but they are flood and drain via auto-siphon. I like no fuss, and little can be less fuss than no switches or timers. Pump always on, water always moving.

Sylvia Bernstein said:

So do you have constant flow through your media beds?

[Dan Brown]

Maybe it's just me, but for those of us using traditional aquarium gravel beds with undergravel filters, the ratios seem skewed (bed:tank volume), because the gravel in the beds isn't taken into account as bacterial media space. I'm going to continue my experiment with the much lower volume of bed:tank (it's about 1:5) and see what kind of results it produces. Once the cycle is complete (still high on nitrates) I think that the additional media will help balance the system. There's a saying in kitchens, "100 Chefs, 100 different ways" which seems to apply appropriately to aquaponists as well.

Also, I'm running 15-15 on the flood and drain.

[TCLynx]

Well if the Nitrates are high, then there are not enough plants taking up the nutrients, a certain amount of space is needed in grow beds for the plants to keep up with the nutrients levels so one can avoid water changes which are part and parcel to normal aquaculture/aquarium culture.

Aquaponics is an art of balancing. Since so many different things affect the balance it is really difficult to come up with mathematical formula that will tell everyone exactly how it will work based only on fish weight or amount of feed given seeing as different temperatures or water quality will affect the bio-filters and different types of plant can take up more nutrients in smaller spaces then others might and different fish types will need different treatment. It is all so variable from one situation to another.

I would venture that recommended stocking levels should be given to get a first year system cycled/and through it's second year for some one who is only learning and doesn't want to risk killing lots of fish by stocking too many or having to re-wash and fill their grow beds for stocking too heavily for the amount of filtration space. Those who are more skilled at it or are willing to delve deeper into the numbers and intricacies will definitely find situations where the stocking rates will be different.

[Kate Mink]

This is a great idea. Maybe call it Rules of Thumb or Handy reference Guide or some such?

I'd like to start gathering up some of these generally accepted rules onto a page that we put up in the top left box of the home page as the third bullet. If no one objects I will start with those above for media based systems...along with the need for 12" or more of media depth. Sound good? Ideas for what we should call it?

[Kate Mink]

Thanks; i was looking more for what the experience of people on this list has been, esp. for the lower-density systems, with just a settling tank or even no processing between fish and rafts. I trust anything that Friendly publishes, still, even they haven't been running and monitoring these as long as people have been working with the high-density UVI systems or the flood-and-drain.

With no expertise whatsoever ;) I'm guessing that established gravel beds develop a biotic community something like soil, and that's how TCLynx can run low fish or fishless for considerable periods. The interesting question is whether this is self-sustaining or runs out eventually.

Kobus Jooste said:

There are two sets of established rules around rafts. the UVI system, followed by Nelson and Pade, worked on a system of fish tank, filtration and then raft. UVI papers are jam full of their stocking rates, raft scaling and such. This was high density AP though. Friendly aquaponics then came up with their ratios for low density raft aquaponics, which is a lot simpler.

Kate Mink said:

What, no one has numbers for raft systems?