pH Drop with Water Change & Gravel Cleaning

Please Help -- At a loss to understand pH drop after water change
I am a newbie to fish keeping & am experiencing a pH drop when I change (and clean gravel) approximately 20% of my aquarium water. My aquarium has been set up for 4 weeks and is still cycling.

Question: What is causing the drop in pH after water change & gravel cleaning?
Is it contaminates in gravel?
Not adding Seachem Neutral Regulator to make-up water?
Please help as I am at a loss to understand the pH drop.

Please Note: I have reviewed the responses to Candgrrl's pH problem. Article was very informative & I will run out today to buy a KH test kit. The responses address adding baking soda & I was wondering if adding Seachem Neutral Regulator would accomplish the same goal?

My current aquarium water chemistry reading as follows:

pH (before water change) = 6.9 as measured by PinPoint meter & Aquarium Pharmaceuticals liquid test kit
Ammonia = .25 ppm
Nitrite = 1.5 - 2.0 ppm (currently in Nitrite spike of cycling)
General hardness = 143
Water temperature: 78 F
Alkalinity: Unknown – Need to buy test kit

I measured the pH of the new water (with Pinpoint meter & test kit) I added to the aquarium & it measured approximately 7.2. The other readings as follows:

Ammonia = .25 ppm
Nitrite = 0 ppm
General hardness = 143

I treat the make-up water with AmQuel (rate of .5 ml per gallon) + and let stand for 24 hours before adding to aquarium.

After I add the make-up water, within hours the pH drops to 6.2 - 6.3 every time. In approximately three days, the pH slowly returns to 6.9

When I first set-up the aquarium, I added Seachem Neutral Regulator (7.0) to the water. I have not added this product to the make-up water.

As to fish I have in a forty-one measured gallons (tank trade-stated size is 49 gallon) aquarium as follows:

4 Panda Corydoras
3 Ottos
6 White Clouds
1 Betta

I do not have live plants nor use CO2 injection.

The make-up water was tested after it was in a plastic (new) bucket for 24 hours.

My best "guess" is that due to my lack of experience & control in feeding the fish and resultant increase in biological waste has created an acidic gravel base. When I clean the gravel, I may be releasing acid trapped in the gravel. Is this likely or even possible?

Would adding the pH neutralizer to the make-up water help?

Thank you,
Panda

Thanks for the quick response PumaWard,

I treat the make-up water with AmQuel + (rate of .5ml per gallon) and let stand 24 hours prior to adding to aquarium. I measured the pH of the make-up water just before I add it to the tank. The ph measures approximately 7.2.

Another thing I notice is the drop in pH before I even add the water to my tank. The pH level directly from my tap is around 7.6 and like I stated, it is down to 7.2 after sitting in a new, clean bucket.

I measue the pH level with a PinPoint monitor and a liquid test kit. The rersultant measurement are within .2 of each other. I tend to go with the calibrated PinPoint meter simply because, as a newbie, I find the color comparisons on the test kit charts to be somewhat difficult to match.

If you do not think it is the KH, what would be another possibility?
When I get my KH kit today and the KH is low, then would you think this is possibly my cause?

Do you agree with using baking soda to raise KH? If so, what would be your recommendation as to measured amount per gallon?


Thanks again,
Panda