Since Wal-Mart sells Triops, I set some up to see what they were all about. I put them in the little 1/2 gallon critter carriers. They lasted about 45 days like the box says and got to be about 1 - 1.5 inches wide. I thought that they were pretty cool and boy can those things eat! And lay the eggs! Dude, I've never seen so many eggs before!
I don't think that they would harm your fish either, and I doub't that your fish would harm them. They grow very fast and have very hard exoskeletons! So as everyone else has said, there a fun things to have for a while!