They wiggle themselves under things, so be sure to check your tank often--and under your furniture if they get out. I found my tesselata under my couch one morning...luckily I saw his tail. My blackedge lept from the net once and got behind the tank stand...that was fun.
I wasn't so lucky with my snowflake. He got out and my dog found him before I did
Just make sure to seal up your tank. I thought I'd sealed my tank up quite effectively until I discovered my tesselata had managed to push the tank lid open and leap to freedom.
If they do get out, and even if they are looking pretty dead, put them back in the tank...they cover themselves with slime to protect themselves when they get out of water. I'm guessing my tessleata was out for a couple of hours before I found him. He was slime covered and pretty crusty...but he sprang back to life when I put him back in the tank. Sometimes you just get lucky!