Yeah, they can be dirty...lol...I once had to keep a pair of them in a smaller bare bottomed tank for a short bit...I could really see just how dirty they are while they were in there.
A neighbor of mine had 2 of the clawed frogs in a large tank...with NOTHING in it but water and frogs..lol..She complained about how filthy and smelly the water always was and always having to to do changes. (what an ugly frog BTW...my first look at those and I can't see why anyone would want one..BLAH)