I read in some other posts about using polyquat. Is that effective to remove black algae easily from the walls? Seems like I may be just losing the battle against the algae although I made huge progress, still algae is not completely removed even though I brushed the same place a 100 times, they are so stubborn. Unfortunately I was sick for few days and still recovering.. so physically I may not be able to do much brushing for couple of days as well.