pH question

[csrede]

What will cause the pH to drop a day after a 15% water change?
I have a one year old 150 gal tank
1 Sailfin Tang
1 Regal Tang
1 Gold Striped Marron Clown
1 Majestic angel
1 Coral Beauty Angel
about 100 lbs live sand and live rock
15 to 20 blue leg & zebra hermits and snails
green star polyps, mushrooms and a bunch of other
things on the live rock
Amm- 0
Salt- 1.024
temp- 78
no2- 0
no3- 20
pH normaly 8.2 but after the last two water changes it has
droped to 7.6 usaly the day after.
ph of new water befor puting into tank was 8.2

 

[broomer5]

My guess would be a very low display tankwater alkalinity.
Have you tested alkalinity ?
Do you aerate the new saltwater batch overnight ?

 

[chevytrks]

definately sounds like alkalinity, do you add any buffers or calcium?

 

[csrede]

The alkalinity tests in the normal range. LFS tested the same as my test.
I used Kent Marine Superbuffer-dKH which has brought the pH back up to 8.2 till the next water change
I usaly mix tne new water 2 - 3 days before the change and have a power head circulating it the entire time.
The new water test and tank water test normal alkalinity and pH8.2 but the next day none of the polyps open and ph is down to 7.6. It usaly takes a couple of days to gradual get it back to 8.2 after which everything is looking fine.
Any clue to what could be causing this? I have not lost anything yet, but hate puting everyting thou the stress.
I also have a 55 gal and a 75 gal tank that I treat the same but have not had any problems with the pH.
Also my 75 gal. tank I only do a water change when the NO3 gets up to 30. It has gone up to 6 months with only adding fresh water to replace what has evaperated. this tank has been up for 4 years with no problems doing water changes every 4 - 6 months.