Fun Facts

Now, a Sparkling Starfish, Green, is available for coins. It's an Algae Eater and costs 5,000,000 coins.

If you want to make that kind of money with White Grunts, you need to sell a little over 35 tanks full of them and sell them individually (by tapping on it and selecting "sell"). If you sell by bulk, it will take 55,7 tanks. This is without buying additional tanks to the one you already have.

Here's the math, correct me if I'm wrong:

White Grunt costs 4200, sells for 5880, but with 91% happiness you get 20% more, which is 7,056. 

But you only get this selling price if you sell individually. 

One tank (50 fish) brings 7,056 x 50=352,800

Minus the initial buying price, one tank yields:  352,800 - 210,000 = 142,800

5,000,000 (cost of Starfish) divided by 142,800 = 35,01 tanks

Selling by bulk, minus 15 % commission: 5,997.6 x 50 = 299,880. Minus initial price, is 89,880. 55,63 tanks for starfish.

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Once you got it, be sure to put it in a special tank so you don't accidentally sell it with one tap, in bulk with other fish. ;)