Just some thoughts..
Must say that i'm having a hard time believing that lowest ph recording of 7.3, even in a room full of people with all windows closed Id never expect to see a tank run much below 7.85 so i'd seriously consider looking at your pH monitor/probe etc as my gut is telling me here that something isnt right with the readings your getting. (maybe a calibration/probe condition issue or the location within the system where the probe is situated.
If that reading is correct (and verified against a second device) then i'd also be very tempted to test your Po4 levels as I'd expect a good degree of dissolution of bound Po4 from any substrates present and from inside LR where bacerial action drives pH proportionally lower.
As per masterswimmer's comments it helps if you take ph readings at the correct times. To find your highest, you need to take it within the last half hour of your lit period, to get the lowest you need to take it within the last half hour of the unlit period. the difference between the two is the important bit, ie minimising the drift over a 24 hour period and this brings us back to your alkalinity which has a direct affect on how much downward drift there is over the dark period (all other factors considerd ie gas exchange etc).
Idealy you want to be looking somwhere around 8-10 DkH which is a bit higher than NSW but allows a slightly higher margin for gradual depletion between water changes on a smaller volume before it drops below the ideal (as in your case)
Allways rember though that if your looking at water chemistry relative to getting a things stable (especially pH) then you need to know 4 key areas that are all linked. ie Salinity, Calcium,Alkalinity, and Magnesium. individually these perameters mean relatively little unless you combine all 4 to get a better picture.
before you do anything your first port of call is salinity, which needs to be around 35ppt on a true seawater
refractometer (or 36.5 if using the more common standard brine refractometers which overead 'true' salinity by 1.5ppt at 35) I pretty much ignore specific gravity these days as its so temperature dependent whereas salinity isnt. Getting the salinity right first, gets all your element levels up around where they should be for testing, so you can then adjust or tweak the main core elements individualy. (ie its no use upping Ca,Mg or alkalinity if your salinity is already low and all other elements are lacking as it will just throw everything seriously out of whack relative to natural seawater).
after you have your salinity correct.
To get Alkalinity up, you first have to have magnesium in the 1300 -1400ppm ball park. With a low magnesium level, you'll struggle to maintain any decent alkalinity level. (a proprietary powderd magnesium addative will usually work well here, but dont increase levels by more than 50ppm per day)
Then work on your alkalinity, getting it up to that ideal 8-10DkH range using sodium bicarb (commonly known as pH buffer, which is a bit confusing as it doesnt actually raise pH, it simply increases your carbonate hardness level which limits pH suppression as already mentioned) but again, dont increase by more than 1 Dkh per day.
Finaly look at your Calcium which wants to be around 420-450ppm relative to alkalinity (you can increae this by using a pure addative like Calcium Chloride) again, raising the level by no more than 50ppm per day.
Water changes will work at balancing things out, but only if what your adding is already correct. If levels in the tank are very low (as in your current alkalinity) it can take alot of water changes to get things back so sometimes its better to manually dose one thing at a time gradually, rather than upsetting the whole system with large influxes of new water each time.
Patience is the key though. so make any changes gradiually and work on one thing at a time over several days or weeks if needs be to give the system and its inhabitants time to adjust to the changes. (as the old saying goes...nothing good happens fast in reefkeeping)
In relation to your current alkalinity reading, this doesnt surprise me at all to be honest. Its very common in the first few months with new systems (especially those using substrates) to suffer a degree of precipitation whilst those substrates chemically stabalise and balance out to the surrounding water. suppression of Alkalinity and some magnesium is very common in such cases.
On the whole, once you have those 4 main perameters in check, 99% of pH issues will dissapear of thier own accord.
Hope this has been of help and good luck.
