I thought I'd answer zlatan2601's (PM to me) on here with a couple of photo's"Are those floating rocks? Or what is it?"
I hung a piece of Ocean rock (Limestone) in a jar of brackish water on a sunny window sill, with a little Alga from the tank side on it, for about 3 weeks untill the Alga started to grow
then suspended it in the tank on braid fishing line, I'd noticed the shrimp swimming to the top to feed on the biofilm then clustering on top of the heater before swimming back down, they soon found the rock
and there's always been a few on it since