More is always better in regards to filtration and oxygenation.
We raise ours in a 300 gallon tank, but we do aquaponics so there is plant material filtering the fish waste out of the pond at all times. We also made our own biological filter that all the water goes through before going into the grow beds. The fish waste has to be turned from nitrItes into nitrAtes before the plants can use them. Also, the nitrite buildup can kill fish if it gets too high (hence the cycling of a house aquarium before you put too many fish in).
We buy ours 100 fish at a time and they do well in the 300 gallon tank. HOWEVER you MUST have good filtration for that heavy of a stock load. Especially if it is fish YOU want to eat, as the dirtier the water the worse the meat will taste. The standard rule of thumb is a stock density of about 1/4 lb of fish per gallon. This would mean approx 100 full grown tilapia in a 300 gallon tank. Tilapia are easy BUT they cannot survive in water less than 50 degrees and therefore MUST have a heater. We buy ours as fingerlings (about an inch long) and they take 6-9 months (depending on how warm the water- the warmer the faster they grow) to reach harvest size which will give you two 8 oz filets from each fish. We tend to stagger our fingerling purchases so we don't have 100 fish ready all at once. We are also working on breeding them so we can raise our own fry (babies).
We are also working with Bluegill right now which you can eat and are quite tasty. We have our own worm farm which we use to feed the bluegill, but tilapia are vegetarians and therefore we grow duckweed in small kiddie pools for them to eat. There is a fine balance between how much you feed them, how fast they grow, and how much waste they produce. Feed to much and the biological system cannot overtake the nitrites as fast causing toxicity in the water.
I looked at a bunch of different types of fish suitable for aquaponics but decided against carp and catfish as they eat anything, which will affect the taste. Bluegill eat only organic worms and our tilapia eat only organic plants, so the meat tends to taste better. If you REALLY like the taste of catfish, by all means try them!
As for fertilization, we fertilize when they are babies and when we don't have fish (or few) in the system, with a standard aquarium plant food fertilizer, (Seachem) We are looking to switch to an all organic fertilizer, but there are few to none that have been tested for use WITH fish, most are developed for hydroponic use and are not food safety tested.
Hope this answers some questions, please feel free to ask me more.
