pH issues...and im stumped

well here is my current problem. besides the aditions of some new corals, (now that my tank is fully recovered from a crash 6 months ago) nothing has changed at all. no new use of products additives or change in matinence or treatment of the aquarium. yet for the past few weeks my ph has been fluxing way to much. starting out at the morning (9am) at 7.9 or 8 and droping down throughout the day to about 7.76 in the evening (usually round 12-1am). usually it stays at a nice 8.21 with an evening low at 8.15. the fish show no signs of stress and the most corals show nothing more than a bit of discomfort (minor shriviling) around 4 or 5 pm which lasts through the night. the ph is being monitored with a professionally calibrated pinpoint monitor and was double checked against liquid tests both powdered and fluid biased. so I'm stumped here. alk is a bit high and so is calcium but it always has been for years, as have i been using the same calcium/alk buffer for 5+ years as well. hope someone out there can pick out something i missed.
My water parameters and tank specs are as follows;
Temp-78.9
pH-8.02 (currently)
Kh in DkH-14.1
Alk-5.03
Ca- 500
PO4-0.02
Ammonia-0
nitrate-0.05
nitrite-0
iodine-0.06ppm
strontium-14 mg/l
magnesium- 1300 mg/l
molybdenum-0.01ppm

Equipt.-
Filtration: Fluval X5, CPR PRO 60 Refugum w/skimmer run by rio 800
2 Hydor Koralia#3 pumps(850GPH ea), about 90lbs live rock and 100lbs live sand.
In Fluval:(3 XL trays) live rock, Renew(3 spaces), phospure(one space), and chemipure Elite(4 spaces).
Lighting: 2 retrofit quad 96w Power compacts, retrofit 175 wt metal halide and 24 hour. lighting system with built in LED moon light, and high power cooling fans.
Other: 300wt Titanium heater with external heat control, Prime 1/10 Hp Chiller(run on rio3100), pinpoint wireless thermometer(reads both tanks at top and bottom levels)pinpoint digital ph monitor,titanium aquarium ground probe.
80 lbs of the live rock from a ten year old system, as well as 45lbs of live sand from same system
Added elements:Kent; Brightwell KoralKolor Tech-I, Tech-CB buffer system,marine-c, and magnesium, strontium, iron when needed. Weiss; blackpowder, marine fish vital, immunovital, coral vital, and spectra-vital.
Food:
Fish; mysis, brine, algae(green,red,brown), spirulina brine, ocean, silversides, whole krill. enriched with Kent garlic-Xtreme, zoe marine , and Selcon.
Corals; phytoplan, Dt's live phytoplankton, and Coral plankton.

Fish: Tomato clown, yellow watchman goby, royal gramma, bangai cardinal, Geometric Anthias/hawkfish, tail-spot blenny, flame angel, sixline wrasse, engineer Goby, purple tang, and Black percula clown

Inverts: 2 cleaner shrimp, hermits, margareta snails, 6+ trochous snails, Hawaiian duster, countless stometella snails, and an emerald crab.

Corals:, Galaxia coral, pulsing xenia, pompom xenia, Xenia elongata, var. mushrooms, green star polyps, snake polyps, finger leather, neon octopus frogspawn, trumpet coral, toadstool(scarcophyton), red cap montipora, neon green monti, Australian Acans, button zoos, Red blue Micros, various rhichordias, Orange and yellow lobe brain, colored zoos, and various other polyps.

yea thought of that one too. did aeration test (dont have money for meter) and all seems fine inside and out the tank. all pumps are working fine and got plenty of surface disruption.

thanks for the link ill check it out

Amphiprion said:

How long did you aerate and did you aerate outside when you did the test? If pH is lower, while alkalinity is higher, it is almost due to CO2 concentrations being higher.

Do you have any sort of refugium? Have you considered kalkwasser?

Click to expand...

you of all people should know how i feel about kalkwasser. but as for the test i tested both inside and outside over 10 hours and compared it to the aquarium. im starting to think it may be the new bulbs even though they were gradually introduced over the last 2 months. i haven even replaced the 2nd 96wt PC yet. last night i checked that my venturi was working properly and that the surface agitation was adequate. there is a small increase in co2. just doesn't seem to be significant enough. regardless i preformed a water change to lower the alk a bit and take general stress off everything. also i picked up a ph buffer that wont effect the calcium or alk. and well see how tonight goes. and as stated in the aquarium specs above i have a HOB refugum.....been to many years i take it? don't remember how i role Amp?

thanks for the welcome back! it seems to be more than "usual" shrinking but only time will tell. once again it is down to 7.82 and the corals don't look happy...but don't look like they are going to perish if i don't do anything. ill give this stabilizer a try in the morning (even though it says it wont cause spikes...lol) and ill let you all know the results.

So once again the ph was at about 7.8 this morning the buffer moved it to a solid 8 but something just didn't feel right. So I retested everything.......results. super low magnesium....my brand new api test kit says its at 800 and my older salifert was testing at 1300 so there is my answer most likely. With old test kit tossed rest soon replaced and a confermation with a test from my lfs...if they even ttest magnesium will tell us all. Please still suggest ideas incase this isn't the answer.