Ph drops during day

[hadams356]

Can someone explain to me why my ph drops during day when lights are on and rises at night when lights go off. It will go from 8.18 to 7.85 in an hour after lights come on and the same when light go off. On 2nd probe and both read the same. 50 bucks wasted. Lol

PARAMETERS

180 gal tank 210 with sump
SG 34
TEMP 78.1
ALK 9.9
CALCIUM 430
MG 1410
PHOS < .25
NITRITES 0

Adding approx 4000 ml make up water a day

Done 3 water changes in last week. 20 gal each

Tank 4 months old, 4 damsels, 2 sps frags, 2 hermit crabs is all that's in aquarium.

 

[Variant]

Are there more people near the tank during the day? If so, that can limit the ph. Is Kalk being added and if so when? Do you have a massive refugium on opposite time schedules as the DT?

 

[hadams356]

Yes on refugium, no on kalk, yes to opposite schedule. Tank in living room, just me around tank. Running skimmer about 12 hours a day, with 3 air stones in sump.