If you're keeping new world cichlids you're better off with 6.5 than 7.5-8 pH anyway. But if it's high from the tap and crashing to 6.5 my first thought is that you've got a lot of waste hiding in there or in your filter and this is causing a pH crash. You should probably deep clean the tank (take out all the decorations, rinse the filter media, etc). I bet that would solve the crashing.
Even though it sounds like your water is really low on total dissolved solids you should get a legitimate GH/KH test kit anyway. Since you're getting these swings it's important to know where these stand. Chances are you can stabilize things with an epsom salt/sodium bicarbonate buffer. Look up the rift lake recipe to get a feel for how much of a difference that stuff makes - ideally you'd add enough to keep the pH stable but without really making it super hard like an African tank.
I would also make sure I had a good pH test kit too. Not just the cheapo strips. Just in case you're getting false readings. I know that my strips are useful only for nitrate levels... everything else varies every time I dip it. But the pH tester I have that takes work (ie, counting drops) is dead on every time.