What is this? Growing like mad!

[Reefman150]

Anyone know just what this is? It was just a small piece on a mushroom rock we bought for the horse tank and never really grew until just recently. In the last three or four months it has grown from the size of a quarter to this! How big will it get? Can I trim it without harming it?

 

[Len]

It's a Rhodaphyta algae, probably of the genus Porphyra sp. I've had these guys in my tank before and they definitely grow like weeds. You can trim them without harming them. Otherwise, they just keep growing