Thanks for the information Kyla.
As I've mentioned earlier my tap and aged water are both pH 7.3. The pH in the tank has always fallen between w/c. Lately it has only fallen to around 7.1 so I've increased the amount of water changed, and also used some tap water that I've just adjusted to tank temperature in a barrel. The fish have not liked that, but rather been stressed. I don't know if it's the larger w/c or the tap water they don't like.
Anyway, yesterday I decided not to give them any food for 24h. Now 24h later I measured the pH and it is... 7.3! So I'm starting to wonder if one problem is that I'm feeding too much. I want to see those bellies full like balloons! I probably need to start weighing the daily amount of food I give them.
I guess this proves what you guys have been saying all along - the tank is too small!