I'm a newbie here, and I wanted to ask a question of the experts.
How do you use the "Isola bucks" in the selling-the-fish store? How do you buy the "Magic Plants" that are found there?
Thanks for any help you can give me.
P.S. Waiting for the last magic fish to grow up!

You don't buy things at the "selling the fish store". That is your store. When you are looking at your aquariums you will see a button on the bottom that says "supplies". You may want to start a second game and choose the option that gives you help getting started.

How 'bout that. So, when you click on the cash register in your selling store, and it says at the bottom of the screen "You have earned 123,456 Isola bucks so far" and click on the plants "You have bought 0 out of 4 Magic Plants", that's just a part of the game? Interesting.
Thanks!

Yes, those are just interesting things to do while you're selling fishies. In addition to various stats, you can get game tips by clicking on the posters.

You actually buy the magic plants and other supplies in the Fish Supply store, as Gatsbi mentioned.