if the water is warm, the tilapia will grow fast, and unless you have some good filtration, the nitrification process can easily get out of balance
and what happens if they do breed and you've suddenly got a couple hundred fry...
for a "small" starter system, you can get plenty of nutrients using ornamental fish, koi can grow quite large (and some are pretty expensive).. the main thing is to use a good quality fish food that provides not only the fish a more complete diet, but your plants as well
i'm growing marmokreb crayfish in several glass tanks (three 55's, a 40, and a couple of 20's).. and in my "single" ibc system (cut in half, 1/2 is the gb, the other is the ft) i'm using just rosy red minnows and crayfish