the best thing to do is to find the magic fish green spotanus or speckled leaffish. You can price them comfortably at about $85 apiece and they survive in the lowest environment level. of course the easiest way to get a lot of these fish is to get 2 of them and mate them with each other. I always kept 2 of each of these in my tank at all times in the beginning to get lots of $$$. there are a number of ways to breed these 2 fish. See : www.cleverarts.com/fishtycoon/ for spoiler help

The first thing to do after that is to get the environment level up. Buy the seahorse too. that will allow more types of fish to survive.