I have that exact same ornament (for my goldies) in a 30 litre tank with a filter, it never gets a film on it like that... but attracting algae!!! OMG!!! always cleaning the darn thing!!! also, I noted that the last time i cleaned it the black paint from the inside was starting to wear off. Now I'm careful with it. Nothing's peeled off in the water though, just when I'm cleaning it.