How many fish

[Joe Bifano]

I have been looking at this for a while and have seen calculators and ratios but mostly for media based systems. I need to get fish for my system and do not want to over stock or under stock and try to get close. I can always add more fish but hard to eat small fingerlings if I have to many.

Here is what I have built. I have 2 systems and I am describing one side and the other is slightly larger but real close.

1000 gallon fish tank with 2 - 725 gallon DWC rafts, 2 - 75 gallon media beds. A total of 2600 gallons.

The system has 1- 4'x8'x6" constant flow media bed bio-filter that gravity feeds into 2 - 4'x4'x1.5' media bio-filters on auto siphons that gravity feed out to the DWC rafts and media beds. So there is about 358 gallons of water in the bio-filters minus the media displacement or maybe 150 gallons. Not growing anything in these bio-filters.

I am getting a DO test strip kit in a couple of days to check this so I can't say yet what I have in the fish tanks and rafts but I have 2 large Sweetwater air pumps going on this setup with 10 - 10" air stones in the fish tanks and 150 feet of soak-er hose in each raft. The reason for so many 10" air stones is 1. I am in Colorado at an elevation of almost 6,000 feet. 2. the tank is 4.5 feet deep, so I have to push a lot of air in this setup along with the altitude.

I plan on growing leafy greens and herbs in about a 6" spacing in the rafts.

I was told on another post to start out with 100 fish per 1000 gallon fish tank to be safe. I was hoping to get 250 in each tank because of cost savings. Will 250 be to many fish? Or is it just better to start smaller and see how things go first?

If I go with 100 how will I know if I can add more fish? How long would I go before I knew that things are working and I can add more? Will I know by good plant growth and no dead fish or are their other things that will tell me I am doing good?

[TCLynx]

How to figure the filtration capacity of different methods of aquaponics varies by the type.  In media based aquaponics, one fish (with a planned grow out of up to 1 lb) per cubic foot of media is a good starting rule of thumb for a new system.  In a low density (read not removing solids) raft system, three square feet of raft per fish (again with the grow out goal of 1 lb) are good rules of thumb.

so since you didn't tell us the square footage on the rafts, it's hard to tell you how many fish they will filter for.

The number of gallons of water in the raft or media beds doesn't really make much difference in this part of the figuring.  If you cycle up fishless, you should have a good amount of nutrients to get you started.

I always recommend starting with less fish.  It doesn't necessarily take that many fish to grow a lot of veggies.  How to know when you need to add more fish.  First, you need your ammonia and nitrite to run at 0 not matter how much you feed the fish and if your nitrates are 0 and the plants showing signs of nitrate deficiency, then more fish might be in order.

[Joe Bifano]

I have 212 sqft on 1 side and 253 sqft on the other side. Using the formula above that would give me 212/3=71 fish on 1 side and 84 fish on the other if all I had were these rafts. Because I have 3 media beds in each side totaling 64 cubic feet does that mean I could have 71+ 64 = 135 on one side and 84+64 = 148 on the other?

[TCLynx]

Yep that sounds good to me Joe.  Once you gain more experience you will be able to adjust your stocking as you learn what your system can handle and what you want from it.  But for now I would say stock between 100-135 fish in the one side and between 100-150 fish in the other side.  If you cycle up fishless first you will already have nutrients for the plants to get started even while the new small fish settle in.  If you just get the fish you may still be in for a bumpy start while things cycle up since normally cycling up with fish should be done by starting with a smaller amount of fish at first to get through the initial ammonia and nitrite spikes before adding more fish and with a large system it can take a while for the initial spikes to happen with small fish but if you stock them all at once and get a big spike when it happens you risk killing your fish.

[Joe Bifano]

Hey TC Have another question on this. Say after 1 year everything is working really good, my fish are doing great and my plants are too and I want to expand.  Using the formula sq footage divided by 3 for rafts and want to add 448 sq feet of rafts, that comes too another 150 fish. That side has 150 now.

Can I add 150 more fish to this tank? The tank holds 1000 gallons.

Also if I keep using the formula for rafts how many fish could the 1000 gallon tank hold and keep them until they are 1lb and water quality safe do you think?

What would be the maximum amount of fish you would put in a 1000 gallon tank provided you had the correct filtration in rafts?

[TCLynx]

Well Joe, I'm not really experienced with Rafts so I don't know that my answer will really apply very well.  Might be best to talk with some one who does do raft aquaponics.

That said, uh... what kind of fish?

The maximum number of fish or total fish weight in a tank is going to be very dependent on the aeration and filtration.  With tilapia or bluegill I would have no problem with 300 fish in a 1000 gallon fish tank provided you make sure there is enough extra aeration to keep them happy and when you start seeing the ammonia or nitrite rise, you either add filtration and back off feeding until you re-stabilize or start eating the fish to reduce the load.

Adding filtration can be as simple as adding a large bin, tank or trash can full of netting and then a degassing method (like another bin or tank with excessive air pumped into it to make sure the water coming out of the net tank can release any negative gasses before going back into the rest of the system.  Or you could add some media beds.

[Joe Bifano]

I have Catfish now and I may add more catfish or bluegill. I have good aeration so far. So if you stayed with these 2 fish what would your opinion be on the amount you might put into a 1000 gallon tank then? I have 150 cats in it now.

[TCLynx]

If you are talking Channel Catfish (or blue or white I think too.)  (If you are talking some type of bullhead it will be different.)

You have enough fish then, Start expanding the system since 1 channel catfish per 10 gallons of fish tank is really appropriate for grow out.  Seeing as you are in a slightly cooler climate, the 150 fish probably won't be a problem for a while and I expect that by the time they are you will be ready to start eating them anyway.

See that 1 fish per 3 feet of raft, was more for the types of fish that you are likely to grow out to just 1 lb.  Our harvest weight for channel catfish is more normally 3-6 lb.  I usually stock 70 fish into a 700 gallon tank.  Sure there are people out there who will get away with heavier stocking densities, I know I have.  But I would recommend that only after you have more experience and can make the decision for yourself rather than needing to ask some one else how many fish is the right amount.

[Joe Bifano]

I am using channel catfish I think. They are black.  I will see how this goes then.

The other concern I have is when I went through the fish less cycle and even now I have very little if any nitrates. I have some algae in the rafts and they are covered all up except for maybe 1" on the sides and that is were I am getting algae. My raft edges are all green as well. In another post you had asked me about the no nitrates and said that something is using it and it must be the algae. The other reason for these questions is if the only way to get rid of the algae is covering up the rafts better and this is as good as it can get. I need a way to suck up the algae so there are nitrates for the plants I want to grow. Or add more fish if the filtration is working too good so I get more nitrates.

I know this is hard to say but what kind of levels should I have on nitrates? The 8 trays I have on the shelves are still not growing. I know that it will take time and that I just added my fish a few days ago but no nitrates.

[TCLynx]

Can you show us some pictures?

How low is the nitrate reading?

I know some people who run with 0 measurable nitrates but the plants grow fine.  Keep in mind a brand new system is going to be slow to really take off, especially depending on the type of plants.  Often it also helps to give a little trace element boost to the plants using something like maxicrop since cycling up with pure ammonia doesn't provide anything but nitrogen and it takes a while for the fish food and fish poo to start mineralization into all the other stuff the plants need.

Patience.  Start feeding up the fish while watching your ammonia and nitrite levels.  If the fish are really small, it could help to get a high protein fry/fingerling food since the smaller fish often benefit from that and so should your plants provided the filtration will keep up and the algae isn't steeling all the ammonia before it gets converted.

Usually I don't think a small gap along the edge of the rafts is a big problem (mind you I don't do raft aquaponics myself) but maybe you could run a strip of something along that edge to block the light a bit more at least until the algae problem subsides?

[Joe Bifano]

The nitrate reading is hard to say with the API kit but it always is below 5 in color and pretty close to the yellow on the chart. I will get some photos put up when I get my little camera working.  The good thing is my PH is dropping from 8.2 to 7.4.  My water temps are cold yet at 60 so I am looking at that and have been asking about raising this up and building some kind of solar arrangement so I can get this closer to 70.

[TCLynx]

Ah, then really patience.  Plants are slow because the water is cool and the pH is high.  As things warm up things should pick up for you.

[Vlad Jovanovic]

I think with the way the dimensions of the Friendly's style raft is set up with the 6' wide DuraSkrim used there's a 3" or so gap. I think Chris Smith mentioned somewhere that he uses (or used at least) some sort of plastic venetian blind strip that floated to block that gap, and that helped take care of his algae problem...

I'm trying to re-work the design (since I don't have american sized 2x4's anyways, or the same dimension polystyrene) to keep the parabola at the bottom corners of the liner that keeps the raft from bowing out, yet eliminate the extra inches that can be conducive to algae growth.