Yes I used dechlorinator (fish's behavior improved after water change).
Ammonia is -0-
CO2 - 35PPM (KH 53.7 , PH 6.4)

I had PH at 6.6 for a while but increased CO2 (lowered PH) to fight algae.
I am going to increase PH to 6.6 as this seems to be the problem.

Any other ideas?

tks