Gallons per fish: does a sweet spot exist?

[Ken Westervelt]

I heard a rule of thumb recently that you don't want to go much more dense than 4 or 5 gallons per full-grown fish.  I assume this has something to do with the amount of nitrifying bacteria available to support the system.

Anybody else have a different rule of thumb they go by? If less, do you need more equipment than a larger DO source?

[TCLynx]

The MAX rule of thumb is around 1 lb of full grown fish per about 5 gallons of grow bed media with a minimum of 2.5 gallons of fish tank (chances are most systems are going to have 5 gallons of water but some of it might be traveling through a sump tank.) THAT IS MAX!!!!!!!!!! If you go more fish than that you are gonna need to start bubbling with pure O2 and other very high end stuff.

Please keep in mind the above is for flood and drain media bed systems. I don't really know the rules for raft systems since the entire system tends to operate differently.

A better number to remember might be the Recommended stocking density that would say 1 fish (with a planned grown out weight of 1 lb) per cubic foot of grow bed media with 1 cubic foot of water to go with it (that cubic foot of water could be split between the actual fish tank and a sump tank if need be.)

(If you think in metric better, Recommended stocking would be 20-25 fish per 500 liter grow bed (that is with a planned grow out of about 500 grams for the fish.)

The thinking here is if you need to ask the rule of thumb, you probably shouldn't be stocking to the MAX. Also, a new system should never be stocked to the MAX.

Again these numbers are for flood and drain media based systems.

Now Nates towers will provide a whole new set of design numbers to play with.

Yes the rules of thumb generally are about the bio-filter and the dissolved oxygen able to support the amount of fish.

If you put more fish in a smaller amount of water, you eventually hit a point where you need major chemical intervention just to keep the fish alive until you can sell them (think bait shops.)

[Ken Westervelt]

Thanks for the clarification. 1 *pound* of fish per 5 gallons of water. That's a much easier ratio to keep track of.

TCLynx said:

The MAX rule of thumb is around 1 lb of full grown fish per about 5 gallons of grow bed media with a minimum of 2.5 gallons of fish tank (chances are most systems are going to have 5 gallons of water but some of it might be traveling through a sump tank.) THAT IS MAX!!!!!!!!!! If you go more fish than that you are gonna need to start bubbling with pure O2 and other very high end stuff.

Please keep in mind the above is for flood and drain media bed systems. I don't really know the rules for raft systems since the entire system tends to operate differently.

A better number to remember might be the Recommended stocking density that would say 1 fish (with a planned grown out weight of 1 lb) per cubic foot of grow bed media with 1 cubic foot of water to go with it (that cubic foot of water could be split between the actual fish tank and a sump tank if need be.)

(If you think in metric better, Recommended stocking would be 20-25 fish per 500 liter grow bed (that is with a planned grow out of about 500 grams for the fish.)

The thinking here is if you need to ask the rule of thumb, you probably shouldn't be stocking to the MAX. Also, a new system should never be stocked to the MAX.

Again these numbers are for flood and drain media based systems.

Now Nates towers will provide a whole new set of design numbers to play with.

Yes the rules of thumb generally are about the bio-filter and the dissolved oxygen able to support the amount of fish.

If you put more fish in a smaller amount of water, you eventually hit a point where you need major chemical intervention just to keep the fish alive until you can sell them (think bait shops.)

[TCLynx]

1 lb of fish per 5 gallons of water but only if you have at least 5 gallons of grow bed to support it!
And in Raft culture, you would need the filtration and a certain amount of raft space and additional water in the system to support that fish. Amount of fish per size of fish tank is only a minor part of the equation.

Ken Westervelt said:

Thanks for the clarification. 1 *pound* of fish per 5 gallons of water. That's a much easier ratio to keep track of.

[Rachel]

So I'm thinking about a 12'X4'X3' pond-- about 1000 gallons. Sounds like that should be able to hold 100-1lb tilapia, but in order to do that I need 100 cubic feet of grow bed? I really only have room for 3- 4'X4' growbeds which I was planning to fill with 8" of media, so that's only 32 cubic feet of media. Does this mean I can only safely do about 32 fish? Can I put some kind of supplemental filter between the end of the grow beds and the pond to make up for this or...?
Thanks!

[TCLynx]

Yes, you should only stock to your filtration capacity. Once your system is well established and you have gained skill keeping fish, you might be able to handle more fish but definitely start with 1 lb per cubic foot of flood and drain filtration.
If you can manage to have the grow beds be deeper, you can gain a huge amount of filtration without taking up more space. The common minimum recommendation is 12 inches deep since it provides much more filtration and plant support. Most of my grow beds are now 24 inches deep and I think that is even better.

If you add on some sort of other filter in addition to the grow beds it can help support more fish but it won't provide space to grow more plants to take out the Nitrates so you will probably reach a point where you need to do water changes to keep the nitrates below 500 ppm. Also, most external filters require some sort of regular attention like flushing, cleaning, back flushing or whatever. If you have another space for growing plants that you might add on later like NFT or DWC you could perhaps use the water from the additional filters to feed that but you can also use water that comes from deep grow beds to feed additional growing space. The additional filters will also take space and not provide plant growing. You can probably get the same benefit by making your gravel grow beds extra deep and you won't waste the space.

Rachel said:

So I'm thinking about a 12'X4'X3' pond-- about 1000 gallons. Sounds like that should be able to hold 100-1lb tilapia, but in order to do that I need 100 cubic feet of grow bed? I really only have room for 3- 4'X4' growbeds which I was planning to fill with 8" of media, so that's only 32 cubic feet of media. Does this mean I can only safely do about 32 fish? Can I put some kind of supplemental filter between the end of the grow beds and the pond to make up for this or...?
Thanks!

[Rachel]

Thanks! I was thinking the 100 fish because that's the quantity that's easiest for me to buy. Maybe I'll just sell half or so on CL. I was planning on the 8" because I read (I think on DIYaquaponics?) that if you go too deep you have problems with solids getting to the bottom of your grow beds and anerobic bacteria. How do you deal with that? I could probably go a foot deep or even 18", I'm just concerned with being able to clean it out. Also trying to figure out if I will need a sump or not. So much to learn!

[TCLynx]

You only have problems with solids build up and needing to clean out the grow beds if you design the system with too little filtration. So, if you design a system with 1000 gallons of fish tank and only 32 cubic feet of flood and drain gravel and put 100 fish in it, you will have to clean out the gravel beds. If you design the system thus, you might be better off adding some sort of solids removal before the water goes to the grow beds. Drawback with such solids removing methods is that you may have to be cleaning them out often (how often depends but some systems require removing the solids from the clarifiers three times a day, others might only back flush and clean filters every three days, all depends on load and design.)

There are plenty of people on Backyard Aquaponics that have flood and drain gravel systems who never clean out the gravel beds. We add some composting worms and use large enough gravel that the flow through stays good even with some gunk and root build up but as long as the balance between amount of grow bed and fish tank as well as fish stocking is reasonable, there is no reason that a grow bed should go anaerobic and need to be cleaned out.

All that said, how you design your system will really depend on your goals.
If you wish to grow out lots of fish and you have no issues with using large amounts of water for water changes and don't really care much about growing plants, then you can simply set up a backyard scale fish farm and do the water changes.
However, if you are interested in the veggie produce as well then an aquaponics system definitely makes alot of sense.
To many of us though, water use is an important factor. My goal is a system that produces a nice amount of fish for us to eat but also requires no water changes because I have designed it such that the plants take in enough of the nutrients that I need no waste water by dumping it and replacing it. I'm lucky, I have a great well and good water so I can get away with wasting water if I so chose. I know many people on city water can't really do large water changes all the time because of the chemical treatments in the water would upset the bio-filters too much to make water changes a good option, especially where chloramines are used instead of chlorine.

In addition, I like my systems to be low maintenance. I don't want to have to clean a filter or flush a clarifier every day. I go out of town regularly for work and I have to leave the tending of the animals to a neighbor. I need to make that very easy for them or I might have lots of trouble getting some one to agree to watch things for me. (Taking care of my AP system while I am away should be as easy as, yes pumps is running, yes there is food in the feeder, no fish floating, all good.)

So to help you design, you should examine what you expect from the system.

Rachel said:

Thanks! I was thinking the 100 fish because that's the quantity that's easiest for me to buy. Maybe I'll just sell half or so on CL. I was planning on the 8" because I read (I think on DIYaquaponics?) that if you go too deep you have problems with solids getting to the bottom of your grow beds and anerobic bacteria. How do you deal with that? I could probably go a foot deep or even 18", I'm just concerned with being able to clean it out. Also trying to figure out if I will need a sump or not. So much to learn!

[RupertofOZ]

I've never heard of deep growbeds becoming clogged and needing cleaning out....but I have seen systems with shallow growbeds suffer from the problem...

Not so much probably due to the actual depth of the growbed... but from the fact that the shallower growbed wasn't compensated by additional beds...Hence the filtration capacity was reduced....

Similarly, systems which are over-fed in relation to filtration capacity... can become clogged....

Although not an aquaponics system as such... I filter my trout system through 210L barrels... two of which are filled with hydroton expanded clay....

With nearly 300 trout averaging 500gms, and a daily feed ration of about 1,5kg.... they remained remarkedly clean...

As TCL says... it's a matter of ensuring you have sufficient filtration capacity for the stocking... and feed rate... for your system...

Many people overstock.... and sadly... often under-filter at the same time...

Stock to your filtration capacity.... full stop...

[dano]

Ive been researching quite a bit and have been seeing one fish per gallon works.I think this is pushing it, but some people have tested different concentrations, and say Tilapia seem to grow faster and are less stressed when they school tightly.
As for everything on the Internet you have to take it with a grain of salt. But we'll see as I'm planning on crowding my fish pretty tight.

[TCLynx]

Pushing the MAX perhaps even with tilapia but if you have 5 gallons of grow bed filtering for 1 fish (grown out to 1 lb) and the fish tank space for that fish could be as small as 2.5 gallons provided there is a sump tank or some other means to control the water level fluctuations for flooding twice the amount of grow bed compared to fish tank.

Now if you are growing fish that are only going to get to about half a pound, you might get away with one fish per gallon of fish tank if they are tilapia and you are providing lots of extra aeration and have the sump tank and all other things needed to support a system pushed to the brink like that. I definitely wouldn't advise that and definitely not in a new system or for a person without ample fish and water quality testing experience.

[ericjf7]

Great bunch of information on stocking levels vs grow bed sizes, tank sizes, and fish per gallon. The next thing that would affect all this also would be how the water circulates, ie. pipe and pump sizes, and how much the various agitation points are adding oxygen to the water through sprays, jets, burblers, etc. - moving water and agitation adds more oxygen at all points. Seeing as how the emphasis is on arriving at a balanced system, there seems to be one item not being addressed; this is the question of consumption. It is one thing to use lbs. per gallon as a measure for ultimate grow out numbers, and stocking levels; 100, 200, or even 300 fry for early stocking levels may be alright in a tank at the early stages, but may be crowded as the fish get bigger - the question then would be at what point do they hit the soup pot or the dinner plates to help maintain that all important balance, meaning that being optimistic with the number of fry may not be a bad thing.

[TCLynx]

It is important, especially with a new system, to start with less fish!!!!!!! Not more. Please don't stock a new system with 300 fry just because they are small, when your system is only designed to say support 30 of them when they are full grown!!!!!!!!!

If you need to be told a stocking density number, you are not prepared to push the max. So if your system is designed to handle say 30 lb of fish. Then only stock 30 fingerlings at the start. The system needs to cycle up slowly as the fish grow anyway.

If you go stocking 300 or even just 100 fingerlings, what the heck are you going to do with them as they grow? And if you go putting that many fingerlings into a new system that is not matured and not designed to take that many adult fish, you are likely to run into problems with water quality and you could well wind up loosing all the fish.

As to maintaining the balance, if you are running a flood and drain media system, stocking level fluctuations are not nearly as disrupting to the balance as one might think. The solids breaking down in the gravel beds tends to spread the load out over time a bit and I've not found that leaving a system even without fish briefly left the plants wanting.

As to flow rates and aeration. I'm all for lots of flow and aeration of all sorts. I fear that so may of the commercial operations growing tilapia has skewed the research to a point that many people try to design system that will leave most real fish gasping at the surface for air. On my large system I'm pumping about 4 times the volume of my fish tank each hour rather than the 1 time per hour that I think of as minimum for tilapia. In addition to the spray bar of water from the pumping through my fish tank I also have a large air stone in the fish tank as well as another in the sump tank. Most grow beds are flood and drain so the bacteria, worms and plants have plenty of aeration too.

ericjf7 said:

Great bunch of information on stocking levels vs grow bed sizes, tank sizes, and fish per gallon. The next thing that would affect all this also would be how the water circulates, ie. pipe and pump sizes, and how much the various agitation points are adding oxygen to the water through sprays, jets, burblers, etc. - moving water and agitation adds more oxygen at all points. Seeing as how the emphasis is on arriving at a balanced system, there seems to be one item not being addressed; this is the question of consumption. It is one thing to use lbs. per gallon as a measure for ultimate grow out numbers, and stocking levels; 100, 200, or even 300 fry for early stocking levels may be alright in a tank at the early stages, but may be crowded as the fish get bigger - the question then would be at what point do they hit the soup pot or the dinner plates to help maintain that all important balance, meaning that being optimistic with the number of fry may not be a bad thing.