This isn't uncommon with sand beds. Phosphates are released and cyanobacteria (the maroon stuff) absorbs it immediately and the population takes off.
When you say you vacuum your sand bed, we need a little more info. Is your sand real fine or is it rough like crushed coral. When you vacuum, do you just lightly siphon off the top or are you vacuuming into the sandbed itself? _________ Curt  |