If i were to put a type of netting to keep them off the bottoms of the FT to prevent babies, how high off the bottom would it have to be? 2"or 3" or do you need more. Also what is the reason for this working?
So long as the netting is a couple inches off the bottom of the tank for the entirety of the cage, then 2 inches should be fine.
This works because Tilapia are mouth brooders. This means that in order to perpetuate the species breeding goes like this. Male creates a nice "love nest" where he clears the bottom of debris and then entices a female to lay her eggs there where he will fertilize them, then She picks the eggs back up in her mouth and broods them there. If the female can't pick the eggs back up, they are not likely to hatch because they need the constant aeration that the female usually provides by pumping water through her mouth with the eggs and tumbling them. So, if the tilapia can't access the bottom of a tank because they are in a cage they are not likely to successfully breed.
I've used fish netting and PVC pipe to make cages before, just have to choose netting small enough to keep the fingerlings from swimming out of the cage while they are still small. There are many types of netting or plastic mesh that can be used for the purpose. Question will be what sorts of plastic are you comfortable with using in your aquaponics system? Does it have to be FDA approved for food contact or are you ok with things that are simply appropriate to aquaculture? You might also want a finer netting around the water line to keep food from floating out of the cage.
I've never worried about eggs that might fall through. I figure they usually got sucked up by the pump or maybe my catfish that were swimming outside the cage ate them. Either way they would be hard to see in a big fish tank.
I've used just fish nets from sporting goods stores for catching fish. Or aquarium nets for small tanks.
Catfish barbs do tend to get caught in some types of netting and the aquarium nets have some times trapped my small catfish if I wasn't careful.
I've used grills to stop fish from going out the plumbing.