You said that you've had them for 6 months. If the tank has been up and running for 6 months, but still has detectable amounts of ammonia (NH3), then that indicates something wrong with the way the aquarium has been maintained. If they've been subjected to this level of ammonia all the time, or intermittently, for 6 months, then I think that would explain the tendency to morph.
The tank looks clean, but this perhaps suggests that it's been kept too clean (which can be just as bad as keeping it too dirty). Try to do some google searches on the subjects of aquarium maintenance, water quality, and aquariums as ecosystems.