Regal Tang Fish

Hi there, I have a Regal Tang Fish which has recently been looking
less than lively! I checked my water and discovered my nitrate levels
were too high. I am now conducting weekly water changes to get the
level down and he is looking better in terms of colour however is not
swimming about - he just seems to sit on the bottom of the aquarium in
the same place. My yellow tang fish is fine in comparison. Does anyone
know if it could be anything else other than my nitrate level? If
anyone can give me any tips on getting him back to health I would be
ever so grateful. Also my little dog faced puffer fish has developed a
little lump on his head!! I have taken some photos which I could
e-mail to somebody in the hope that a puffer fish expert reads this
plee for help. Many thanks. Thomas in Leeds.