I have moss growing in mostly all my tanks, and I did not put it in any of them. You can buy java moss, rica (my favorite groundcover), and kyoto moss. Java and rica grow as long as there is water and decent light, but kyoto is a little finicky. My moss usually starts to appear after about 3ish months and really makes a nice carpet by a year.
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A room of herps.
www.freewebs.com/kapoktree