Double money and glitch
Buy food brick and one at a time buy green snapper for $10 and let it feed for $1 back so it only cost $9. Then sell one at a time for $18 if you have them at 91% happiness or above.
You can breed green snapper $10 and clown fish $20. Green clown after a while is worth $1601 + extra for happiness.
Keep fish alive
To keep ur fish alive without reviving them You'll have to put all of them into inventory and when You're back just let them out.
Tap Each Tank Seperate when you return from a period of time. Also feed new fish individually after you buy them. You will get 1 coin every time.
Money Cheats / Glitches
Keep buying Asia arowama I think it's spelt then when u have about 100k+++ then buy the Cromis or golden dwarf which I recommend. Keep buying 50 golden dwarf per tank then u can get a whole lot of reward after one day.
White Grunts are the next step after Golden Dwarf, about 200K per tank, and after that your on to Barracuda at around a million coins per tank, but with big profit margin ... especially if your fill your tanks with fish happiness decorations) for a hefty bonus. Pushing 91 happiness seems to be pretty good for me.