one problem could be acclimation. seahares are molluscs which means they're VERY sensitive to salinity changes. The drip acclimation process is the best for them.
here's how:
- put your new animal and the water from the bag in a clean container (tupperware will do but something insulated is best)
- set up a slow drip (2 drops per second) for an hour
remove half of the water from the container and increase the flow rate to a slow trickle
- remove half the water a half an hour later and increase it to a fast trickle
- test the salinity. if it's the same in the container and your tank, add the animal to your aquarium. if not, continue acclimating at the current trickle rate until it is.
this acclimation process is good for fish too but add a little Prime or some other ammonia neutralizing chemical so that the fish doesn't poop itself to death during the process. (ammonia chemicals are not nesessary for inverts)
also keep on top of your evaporation. dumping in lots of top off water at once is hard on snails n such because of the sudden salinity change and may be the cause of a "snail graveyard"
if you're running out of food for your sea hare you can feed it nori