Well, like the question says, which is the best fish to breed at the start of the game to quickly get some money together to buy the research etc?

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.

When I start a new game I try to buy as many batches of common fish eggs as possible. I always end up with at least 2 green spotanus. Then just Like the last post says they survive in the lowest environment and sell for a respectable price In The last tank I started I didn't even try to breed anything other than the Green Spotanus until all of my research was done