7.8 to 7.9 at day or night? Check diff photo-period & check from main tank.. if night and young system, shld be marginal but not really bad yet. Day shld be >8 at least.. Drip kalk during night may helps..
do you have any carbon media or any acidic filter? Try remove them for the moment. Try remove any died to avoid this? Also, depend how mature is the system.. Normally young system tends to tendency of PH swing.. As long as not over populate yr tank shld be fine.
Do not stir your sump. May have adverse effects...
Cheers and Happy Reefing..