I don't have a filter in my 5 gallon amphibian tank (*ducks as rocks are thrown at her*.....). I do regular, large water changes (70%, twice a week), plus I have plants that help suck up all the nitrogen in the waste products (I have some Windelev which I am rooting to the log, which of course is fully aquatic, plus a pothos cutting, which is rooted in the water (big ole' roots!), but is draped over the log so the plant itself is out of water for the newt to hang out in).