I posted a photo a while back on the identification thread of what I thought might be some sort of Jelly fish, which was confirmed on this board. No one seemed to think that they were harmful an any way, and they never got any bigger due to the fact that the skimmer and filters killed them off before they could reach adulthood. Since I found them fascinating, I caught a few and moved them over to a 5-gal
pico tank that has less water movement and have been feeding them. The largest one is about an eighth of an inch across and is reddish with purple spots.
I have other photos on my blog of the jellys and other tankmates if anyone is interested at Weird Things In My Tank