I guess it might depend what water and additives you are using, if you are using straight tap water you might expect it to get limescaled quite quickly, you could flush with vinegar or CLR or coffee machine cleaner to clear that.
If you are using distilled water but have open reservoirs then you might get biological contaminants sliming the thing. If you have a sealed system with distilled water and antifreeze or something, you probably don't need to clean it.
Road Warrior