Now for a long time many people have said you could stock an outrageous amount of fish in an aquaponics system, however, I think most of these claims have some numbers mixed up and are probably taken a bit out of context, then when some one new to aquaponics goes and tries to mimic what they heard it can get even worse.
Now add to that, most backyard aquaponic hobbyists are not going to weight their fish to figure out how much they should be feeding and we really don’t want to be pushing the limits to the point that an hour without the pump running is going to cause all the fish to die well lets take a step back and see how much fish is really needed to power our aquaponic veggie gardens.
I know of some systems that have very minimal amounts of fish and produce lots of great veggies. I also know of systems that have huge amounts of fish and produce very little in the way of veggies. Please don’t get too hooked on the idea that you have to feed some specific weight of food to your fish each day in order to support your plants. Those numbers came from a very specific system under very specific conditions. They remove the solid wastes from that system daily if not several times a day. By removing those solids they are removing most of the nutrients from the system before mineralization and therefore before the plants can make any use of them. In most backyard type aquaponics systems where the solids are being left in the system for mineralization in gravel beds, it doesn’t take nearly as much fish or fish food to support a huge amount of veggies.
Now eventually a lower limit of fish and food will be reached when there won’t be enough nutrients for the plants but I don’t know what that limit might be. I have left an aquaponics system for months without fish or fish food and the solids breaking down slowly in the beds kept the system in enough nutrients for the plants for a long time. One still needs to be careful when re-introducing fish into a system that has been without, the bacteria population will have declined and cycling back up will be necessary.
In my large system at the moment, I have 70 fish in a 700 gallon fish tank and there are about 1400 gallons of gravel bed in that system. So that is 1 fish per 10 gallons of fish tank and 20 gallons of media. Granted I grow big fish but I’m certain that small fish at those numbers being fed a high protein feed will be more than adequate to keep that system in nutrients.
Currently the 300 gallon aquaponics system has only 14 fish and they haven’t been eating much after the transfer and that system still has high nitrate levels so no worries about running out of nutrients there either.
So I think I’m gonna recommend about 1 fish per 10 gallons of fish tank for my basic system designs for the first season (after initial cycle up which I usually recommend done fishless.) Then following seasons I expect that the fish load could be reduced if the family finds they are not wanting to eat so much fish or elevated if they want more depending on the system performance.
What size of fish is the 1 per 10 gallons ratio based on? I’m having a hard time getting my nitrate level above 3 ppm w 70 mixed size tilapia in a 150 gallon stock tank and 450 gallons of DWC. It’s a new (3 months after cycled) system and the plants are doing fantastic, but I figured there would be more nitrates provided by that many fish. Ammonia & Nitrite are both at almost 0 too.
Well, my 1 fish per 10 gallons is for a media based system. In your raft system are you removing solids? What % of protein are you feeding to your tilapia?
Then again, if you have a measurable amount of Nitrate, then the plants haven’t used it all so you have enough. I’m actually wishing I could keep my nitrates down below 20 or 30 ppm since it gets hard to read the colors above that.
As to the size of the fish, well they are small to advanced fingerlings when I put them in the system and they grow out over a year’s time to be from 3-6 pounds or more. I grow channel catfish which can take the heat and the cold (provided they are not tiny when it gets cold.)
If your fish are still small, you might want to be feeding them a fairly high protein feed which may increase your nitrogen levels. But as I said before, if you have measurable nitrates, then you have enough. It is only if nitrate is 0 ppm and the plants are showing signs of nitrogen deficiency that you can say you don’t have enough.