its hard to say what the max price for each fish is. The best thing i found to do was either breed magic fish and sell them for money or breed orange fruit fish. i think sell for around 20$ a piece. Get a tank full of those and you can do some good. But research is key to survival. Until you can breed more exotics, there isnt much you can do from my experience. Try to get yourself two magic fish of fertility and sell them at higher prices and you can make some money. If you want to know how to get it just ask, didnt want to spoil it in case you didnt want to know the secret to getting them