Clownfish sitting at bottom of tank

[SeaSc0rr]

Pretty well established tank has been running for well over a year, but just added a pair of clowns not even a month ago and one is already having problems. All other live stock, snails, crabs, and the other clown are fine. Though the smaller clown in the pair is having troubles, sometimes catching him laying at the bottom of the tank and breaking very heavily. Couldn't get a picture because whenever I walk up the tank he comes up to the top in hopes of food. Don't think its water parameters I keep up with changes weekly and like I said the other clowns alight. Any ideas? Thank you

 

[SeaworthyAquatics]

Any other symptoms? Scratching, discoloration, spots etc? Anything else added since the clowns?