Just bought the game, been playing for 19 hours and have 11 species of fish. I know that in the beginning if I cross-breed fish, the babies die because the enviorment isn't good enough. But I don't have enough money to research enviorment.

Should I
** don't buy anything, save money until I can research enviorment

or does buying the cheaper things help, like the aeration system and temp regulator etc? If I buy those items, will that improve my tank enough to breed better fish?

Everyone has a different strategy... but for me, I keep from buying anything until I can buy enviroment. If you breed Golden Goldbulbs, they bring in the fastest cash and will live long enough to sell them. Once you have better environment, you'll be able to breed the high dollar fish!

There are at least two other fish that sell for considerably more than the Golden Goldbulb, and one of them doesn't lose health at all in a Level 1 Fish Environment. It usually takes a bit more work to get them, but I think it's worth it.

I guess what I am asking is do the aeration and temp controls and those other items improve the enviorment enough to make it worth buying them? or should i only purchase the actual enviorment research?

These things do help a bit, and because of the low investment, are ok to buy early in the game. Sometimes I buy them immediately, but usually I don't.