My tank has been running for over 9 months now, FOWLR 150 litres, the last few months I've been having trouble with what I guess is brown algae on my sandbed. I'm changing 25% of water each week hoovering the sandbed (which removes it all) then the next day as soon as I turn the lights on within a few hours it's back. Readings seem perfect Amm 0, Nitrite 0, Nitrate 0, pH 8.0, I'm using chemi pure elite, purigen, have a pellet reactor, i don't overfeed I just can't understand what I'm doing wrong - please help!