as far as i know they are good for your tank as long as there is alage for them to eat . but they will consume it quick and then die. if you can i would take it and trade it to the lfs JMO
I think a 29 gallon tank would be too small for a sea hare. They eat a lot and they eat fast. It would probably starve pretty quickly in a 29 gallon tank. If you are having algae problems, try a few Mexican turbo snails instead.