You can increase O2 levels by increasing surface aggregation, ie aim filter outlet at the surface.
KH can be increased by using Sodium bicarbonate(which your mum should have).
Could of saved quite a bit of money!
If in doubt do a water change.
Dats sometimes decide not to eat, well thats they way my 5 behaved.
PH of 6 is fine as said above, as long as its stable you dont have any problems.