I'd been thinking about this one too, but the only way I could think it could be done was to have a fish "stockroom" where you say how many fish are "in stock" of each type, which the "random" feature could then choose from. I like your idea much better - it's really quite simple and also "visual", with the sliders.

(You would still have to tweak the solution to only allow seven sliders in the "always" position though.)

Now, if you had that and a "refresh fish selection every __ minutes" option, that would be brilliant !