Apologies if this has already been suggested.

I mostly just have all 7 fish set to random, however there a couple the of fish types I don't particularly like, and it would be good to have a way of stopping these from appearing as random fish.

I'm thinking along the lines of a set of check boxes where you could mark each of the fish types as 'active'. Taking a fish off Random and setting it to a particular type could over ride the checkboxes for that particular fish. This would allow you, for instance, to ensure that you only had one of a particular fish type, while allowing the rest to remain random.