Do you have substrate in this tank? If not, swish all the rock around in the water which will loosen any dead debris. Once the debris has settled, siphon it out and do a major water change, say, 30-50%. If you have any mechanical filtration in the tank, clean it thoroughly in the outgoing changewater. If you have a sump, siphon out any debris from it as well.
Once you have all the crud out and the water change performed, test pH just before lights on and just before lights out. This will give you your daily pH swing. If the swing is over .2, for instance 7.8 to 8.0, you will want to drip kalk into your system each night to to keep the swing from becoming larger.
The washing soda idea is a good one. It will raise pH, but will also raise your KH. You are aiming to have a KH between 8 and 11 dKH.
If you get your KH between 8 and 11 dKH, and your pH is still low, I would say you have high CO2 levels in your tank room and the only way to raise pH in this instance is to get more fresh air, which will contain O2, into the room. High levels of CO2 will depress pH.
While you're at it, test calcium levels. You want to have about 450 ppm calcium.
More test kits, you ask??????!!!!!??? Yup
