Planted Tank Enthusiast
Join Date: Dec 2011
Location: San Antonio, TX
I can actually help you answer this. I had them in my discus grow out tank and couldn't find out what they were! After about a week of online research and asking around, I found out that they're a type of freshwater Bryozoa.
There are tons of different kinds and in aquariums we don't get those huge solidified colonies like they form in the wild (at least I hope no one does - yuck). Their life cycle is about 5-6 weeks and they like oxygenated, food rich, well flowing, well lit area. They can come in on water, plants, fish, anything. Imagine them to be like BBA. They aren't harmful to your fish or anything. They don't sting like hydra either.
I believe mine came in with my discus. Feeding them varied foods 6-8 times a day, sponge and hob filter, large water changes everyday, it was a good breeding ground for them in that tank. After I moved the fish, I bleached the tank and filter, got new media and restarted the tank. They showed up again. Snails don't eat them, neither do plecos, I haven't tried shrimp. I tried dewormer like one does with hydra, no effect. There's nothing living in the tank now, but they'll still send out their eggs/spores into the water. So it's not from lack of water changes or over feeding. That's all I ever came up with *shrug* I hope it helps. Maybe someone else can add any info to it. I posted about it in the "bug" sticky when I saw them.