Clown Fish lying down

Question
Thanks for the reply. I think we figured it out. The guy at the local fish store asked me if I make my water up before hand. At that point I didn't. He thought that the salt wasn't all the way mixed and the clown ate some salt and burned his gills. He labored for a few day but has since recovered and seems to be doing fine.
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-----Question-----
Our 12 gal. tank is 4 weeks old w/ 2 ocellaris clowns. Introduced a small damsel yesterday and did a partial water change today. We also rinsed and "repositioned" some plants too while cleaning the tank. Now one of the clowns is lying down on his side and breathing seems to be laboring. What did we do wrong? Should we be concerned about its health? ALL levels in the tank are checking out okay.
-----Answer-----
How long have you had the clowns? Is the other one showing any signs of distress? How do you prepare your water to add to the tank during changes? Please get back to me and I will try to help. Sometimes, by the time you find a fish in stress it can be too late. Try not to feel as though it was your fault. Gerry.

Answer
glad to hear things are better. Premix your solution three days in advance with a heater and an airstone. The water will be readily accepted with no harsh reactions. Make sure salinity and temp match so the new water will enter the tank with the same basic chemistry as the tank water.