What works for me is to put the starter eggs in the tank, spend the money to buy another set of common eggs and put them in the tank. Once the fish are grown, you should have a couple of fish of the same kind that you can breed and have the babies live. Keep breeding them (and their offspring) to build up cash. I usually save up enough money to buy the second tank first. That gives you a lot more breeding room and you start making money faster. The trick is to be patient and not spend all of your money too soon.