While I have found that at the start if you get A Silky Spotus, Twin-Fin Shark, Orange Fruitfish, Greenfin Spotus (wich is magic) you can get it in your first set of fish most times. Also Silky Stickfish and some sort of Fatfish. You can get a few fish that are worth $25-$40 and if you dont buy anything except medicine if needed then you will quickly get enough money to upgrade to the next level for the Fish Enviroment. Then you can keep those new fish you have gotten Alive long enough to breed. After you get the upgrade get and Airfilter from the Supply shop and then the food Upgrade and then save for the next enviorment upgrade. So here are a Few Combos:

Silky Spotus & Twin-fin Shark = Golden Glub Price@ $40-45

Orange FruitFish & Silky Spotus = Pink Leaffish pice @ 20-25

Greenfin Spotus & Twin-fin Shark = Flagged Glub pice@ 25-30

Thats all I have for now I will update this as I have more if anyone has any other eaisy combos to get fish that can get decent money please feel free to post.