Lump On Tetra; Swimming Erratically - Help!

[jensen]

I'm relatively new to aquariums and I'm very worried about one of my fish. My serpae tetra has been swimming erratically, strangely enough, ever since the earthquake on the east coast of the U.S. several days ago. He tends to go over on his side but can get himself right side up again for a period of time. Today, a lump popped up on his side, right near one of his black spots. I tested the water and it tested the way it normally does. I change 20% of the water every week, and add only a water conditioner. There are 2 other serpae tetra, 3 black neon tetras, and 1 blue neon tetra. They have all lived together for about 5 months successfully, and the other fish are acting normally. Is there anything I can do to help?

(parameters are from quick test strips)

Tank size: 10 gallon (I know my tank is a little crowded - working on getting a bigger one)
pH: between 6.2-6.8, which is normal in my tank
ammonia:0
nitrite: 0
nitrate: 0
kH:0
gH: 75
tank temp: 76 F

 

[jensen]

I should add that I recently changed my charcoal filter and the water is a little cloudy. This sometimes happens when I change the filter, but clears up in a few days.