need advice on ph


somewhat experienced newb
hi, I recently received a ph moniter and after calibrating it I installed it and it was reading 7.6 - 7.7 . so I double checked with another test kit and it looked pretty close. my alk was around 9. I added some ph increaser and it raised the ph to 7.8 so I added more and it raised to 7.9. next day I checked it and its 7.6 again and my alk is now 13. now I don't know what to do or why it keeps dropping. im thinking im going to have to do a water change to lower my alk. all my coral look great with the low ph which I thought was strange. I always keep my ph around 8.2 and have never had a problem before. only difference is I turned the furnace on in the house and theres a natural gas wall heater right outside my filter room. so im thinking its a co2 problem. any suggestions would be greatly appreciated. thanks.