Over the weeks I've been seeing that my tank has this white stuff develop under the filter (behind the spout of water) next to the surface of the water on the tank walls. I clean it off but some two weeks later it reappears. I've never put much interest in this until something hit me in the head to ask here.
First off, what is this white stuff? Second, is it threatening, or does it ring a bell that something needs to be cleaned?




