You can spend a lot of coins trying to figure out what to feed your Aqua Pets fish so that they'll give you a present of coins the next morning. Sadly, the number of coins they give you doesn't seem to be related to how common or uncommon the fish is. Bigger fish give more coins than little ones... but the downside is that three size 1 fish give about the same amount as one size 3 fish.
Best strategy is to go with one category of fish (carnivores, freshwater, etc) per tank and use slightly larger fish. It's fun to watch the little ones scooch up into a feeding frenzy, but the advantage of going with larger fish is that it's easy to make sure you've fed everything in your tank. And they won't give you coins unless they've been fed.
Hands down, the easiest thing to focus on when you're at levels 1-6 is to keep all the freshwater fish you get (caught with the original fishing rod.) I've made a list of the ones I caught with "black bugs" and "orange bugs" and as long as you keep fishing with the wooden rod, you'll fish up enough of these to make a tank.
A list of fish for your first "money tank":
* White and red Dragon Betta (rare)
* Golden Arowana
* Green & Yellow Betta
* Lime Green Guppy
* Rhinogobius brunneus
* Black & Red Koi* Blue & Red Betta
* Brown Catfish
* Pink Koi
* Red Cap Black Oranda
* Red Cap Pink Oranda
* Red Ryukin
* Silver Arowana
* Telescope Eye Goldfish
When in doubt, google for the name of the fish (if it's a real fish). A little "freshwater tank" is something you can fish up by the time you're level 5. Feed them "Marimo" (green blobs, near the bottom of the food list) and they'll be quite happy with you.
Things that like "Chicken Nuggets" include: