Corydoras Problem

[Tracey73]

One of my peppered corys seems to be having trouble staying grounded. He is getting around ok and eating but soon as he pauses for a split second, he starts to float upward :( I have had him for 1 year, water parameters:

Ammonia 0
Nitrite 0
Nitrate 5
Ph 6.8
temp 29

I have 8 other corys that are perfectly fine!
Any help much appreciated :wink:

[Tracey73]

my cory has gotten worse, he is now floating at the top and not looking good at all.. water parameters are the same as above.