These brown stuff just started appearing out of no where. This isn't the first time that it happen..it's the second time. First time it happen I didn't know what it was so I just thought that it needed just cleaning only so I took my 4 cories and my plakat out of the tank and cleaned it. It was going good for awhile and now it just appear again. It's on the side, the heater and the decorations. Please tell me what it is. I don't know what the problem is.