I think most people want extra tanks because we don't want to have to get rid of our favorite fish. I know that after beating the game the first time, I continued to breed fish, trying to make all 441 of the lil buggers. I also know that giving us access to more tanks would make things really easy.
As I see it, there are three possible solutions: make additional tanks incresingly ridiculous in price, OR only available for purchase as the number of species discovered increases, OR make them available only after the game has been beaten.
I also like the idea mentined earlier about giving the user options in the decoration of the tanks. Maybe the first tank could be a standard starter tank. Then when additional tanks are purchased, items are unlocked for purchase. This would allow the user to change the color of the rocks, decor in the tank, and even new backgrounds as the game went on. It would be really cool if there were a funtion where people who aren't tech save can "upload" their own backgrounds and decorations. I do think that the items should be unlocked slowly as certain events in the game occur, such as breeding the magic fish, discovering key numbers of species, or having sold key numbers of fish. By keeping the others greyed out and unavailable, it sets new goals for players. Also, decorative items should be available for purchase more than once incase we want a ship in more than one tank.
I love the fish, even though the lack of real size differences kinda bugged me sometimes. A shark and a carp the same size? Oy. I think it would be cool if some of the fish, while being the same species, came in different colors. Or have them different species with a slightly different breeding pattern. Like if you bred a Twin-Fin BlueGourami and a Silky GoldGourami together, you would randomly get either a Golden GoldGourami, or a Golden BlueGourami instead of some comepletely different type of fish. But if you bred either a BlueGourami or a GoldGourami with a Shark, you would still always get a Fatfish. That would make it a little more challenging for people actually trying to make their favorite tank.
Someone mentioned the idea of being able to sell other items while items while in the shop waiting for your fish to sell. I like this idea as far as having something to do, but it shouldn't be a major source of income. Maybe getting an extra buck or two for pushing fishfood or meds. I'd rather see a variety of little puzzles though; they could either earn you a few extra bucks, speed up the speed at which your fish sell, increases the price range where your fish would sell, or bring in more customers. Or fish trivia, even.
Thanks for listening.