I've got an update ready to go for Aquarium that will let you set a fish as 'random'. Basically, the fish selection window now has a "?" option, which will let you set that particular fish to change its species occasionally. This means you can choose how many random fish you want.
There's some loading/unloading on the fly going on here so I'm going to run it for a while and make sure there's no problem, but it seems to work pretty well. Here's a pic!