Black Neon Swimming Sideways

[dusty2]

Hi All, Please can you help me. I have just come home to find one of my Black Neons swimming sideways. He does apear to be able to move quickly when another fish comes near him but then swims on his side again. My tank has been set up for around 7 months with no loses to date. I have been changing 25% of the water on a weekly basis. Any advice would be appreciated.
John.

 

[WILDER]

How many gallons or litres is the tank.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.

What do you feed the fish.
Does he look bloated.
Does it look like his tail weighing him down in the water.
Does the fish turn upside down.