I pretty much agree with the rest of the posters. Yellow(lake) perch can top out at 14+". I have caught 13" myself. So, the 20l will last only so long. I agree with a 75g tank minimum. That would allow 5 full grown perch. Don't mix your natives with other fish and don't use a heater. As for food pretty much what JRR stated, I would get your food( worms, minnows, etc.) from a bait store as they will be much cheaper than an lfs. They will grow faster in your tank than in the wild due to the warmer water increasing metabolism and the fact that it will stay constant they will continue to grow constantly, as opposed to being in a natural lake where seasons affect growth rates. They are neat fish to keep,especially with a school since you will witness their natural behaviors more. Also, IME I would not keep them with other natives such as bluegill. Perch are alot less aggressive than gills and will have a harder time getting to the food. I did notice that when I kept them, their colors would change depending on their mood, being more vibrant when hungry and more pale when fed and content.