I have a 4ft tank been up and running a year and has an external filter.I do 40-50% waterchanges weekly. the past 2weeks the sand has algae on it.there is no algae anywhere else on the tank exccept a few tiny bits on the bogwood.the algae just sits there like a green dusting,it goes when i clean the tank out but is back the following day.what can I do?







