One of my eight Chen discus is just not quite right. For the first few days after getting them it was one of the most confident and fed well. Since then it has withdrawn to the back of the tank and rarely comes out. It does pick and eat beefheart & brineshrimp and its faeces are dark and evident most days but something is just not quite right with it. It holds it colour like all the others and is for all intense and purposes a very healthy looking fish, but compared to the others its just not right. Is there anything I can do? I'm beginning to think it my have a digestion issue?

I change 40% of the water each week using RO water and the parameters are Temp 29C, zero Ammonia and nitrite and 20 nitrate. Gh is 8-10, kH 4-5 and pH 6.5.

Anything I can do to get the fish on track?

I've had them all for two weeks now.