I used the starter fish to breed til I found a fish whose base price is about $20 (that's probably the highest price that lvl 1 environment can handle), then I breed a lot of that kind and sell most of the others (keep the parents so you can use them to make more). Then I set the price up a couple bucks. If that sells within the first couple min, I go back and up the price a couple more bucks. And then Repeat. If I sit and watch for a few min without a sale, I walk away for 10 or 15 min. If they're all sold, I up the price again for the next batch. I got my orange leaffish from 20ish to 41 over time, and used it to fund lvl 2 &3 environment upgrades, the second tank, etc. Then I bought higher lvl eggs to find something I could sell for more.
Hope that helps!