We should be able to have at least three fish to the regular size tanks and larger. The tanks take up way too much room and don't look realistic with only one fish in each tank. Another nice addition would be the ability to craft our own items to put in the fish tanks. Items that have a theme to match the worlds, the fish, or both. If we were able to decorate our own fish tanks and put three fish to the tanks it would look like it fits in the house and not so out of place.
Instead of a fish tank, I would love a flat screen tv or tapestry that acts like a fish tank I can forever add fish too; this way every time I walk by, one or more random fish would appear. It would be cool if visitors could even select which fish they could also check out in detail too.
I would appreciate fish tanks that hold more than one fish. I'd also like it if a title or name of the fish in the tank appeared on the outside of the tank; just so everyone knew the proper name of the fish. Expanding this idea further, when a player moves their cursor over the tank or fish label, a pop up appears that provides a complete description of the fish, where wizards can fish to catch them, and what recipes the fish is used in. It would make the fish tanks more interesting.