There are a few good fish to aimk for early in the game.

With very little research/purchases (I think you can get away with maybe the temperature or air bubbles purchases only) you can breed a whole tank full of one rather handy little fishy that will sell for around $35-38 each. That can really boost your bank balance early on.

As a quick clue - this fish is what most people might consider to be the archetype pet fish.