7.8 or a little lower is not too uncommon after lights out for tanks not dripping kalk
at night. I run at about 7.8 at night even w/ a fuge full of macro-algae on a reverse lighting schedule. However after the lights have been on for a few hours you should at least hit 8.0 ph IMO
. I think the next step would be to take a sample of tank water outside and put an airstone in it. Check the PH after an hour or so. If it goes up substantially it's most likely the air in your house. If not, the tank/LR
may just need some time to mature.
FWIW a tank generally is not considered mature until a year or so after the initial set up. Let us know what happens w/ the sample.