The water chemistry is fine and I have no idea what else it could be. (Ph-6.5,gh&kh-4,temp 29 degrees).
Sorry that you are seeing issues with your fish.
If you have a pH of 6.5 and a kH of 4, then you should post the actual ammonia, nitrite and nitrate readings. Also you should post the pH of the water out of your tap or well.
Also, it will help if you post the following:
1) What recent diseases for those and other fish in that tank have you seen, especially if you had any recent deaths?
2) history of those fish (when did you get them)?
3) history of that tank. When was it set up? How long has it been fully cycled?
4) What medications have you already used if any?
5) Are you using test strips or liquid test kits? (Strips are notoriously inaccurate and while liquid kits are more reliable, it's important that they not be expired and that they are used precisely as instructed.)
The more accurate information you give, the faster and more accurate response you will likely get back.