The good news about a dropping pH: the ammonia is dangerous above a pH of 7 or 7.2. (It turns to ammonium below that.) It is even more dangerous at a higher pH though.

In the realm of chemistry I really feel out of place. Ask anyone inclined toward chemistry if what is mentioned here sounds reasonable or correct.

If your pH isn't plunging don't panic. Keep up the water changes.

Buffering Capacity = the ability of water to maintain a stable pH. It is controlled by the amount of carbonate ions present in the water. It is also called Alkalinity.

"°KH - Buffering capacity, temporary or carbonate hardness. The "K"; in KH comes from the German word 'karbonate'. KH is a measure of bicarbonate and carbonate ions that act as buffers to prevent the pH dropping. The ions that make up KH can be removed by boiling. KH makes up a component of GH, so boiling will also reduce GH slightly. One degree KH is equal to 17.9 mg/I CaCO3. It's also measured in degrees. The degree symbol may be replaced with a d (ie. 2 dKH). Because KH makes up part of the GH value, you can't have a KH higher than your GH level."

http://www.mn-aquarium.org/masartax.htm

There are commercial products which will buffer pH up or down. They might be dangerous if used apart from water changes. Ironically, if used with water changes, the pH will still change some. You could use some to raise the pH, but keep an eye on the water. Better might the decision to remake your water or leave mineral materials in the tank, to buffer it.

Your alkalinity is not as low as PeterW's. He may have some insights here

Ah! The bubbles I was concerned with were in your photo at http://img.photobucket.com/albums/v610/lolonque/dory.jpg

The point was not that your airstone was the essential problem, but that the surface bubbles, made up of something in the water, were threatening the oxygen exchange at the surface of the tank. I did a poor job of suggesting that those bubbles must be removed. Aeration shouldn't be removed or scaled back.

More current from an airstone or filter wouldn't be bad. What was unhealthy was the refuse collecting in those bubbles which needed to be removed. In an odd way, they resemble the bubbles marine aquariums have in protein skimmers.

Monitoring the pH alone allows you to detect when a change has taken place, but it provides no information regarding the stability of the pH in the tank environment. The dissolved mineral content (alkalinity) affects the ability of your water to hold its pH values steady. Those minerals which directly affect the pH stability (buffering capacity) of your water are: carbonates and bicarbonates of sodium, potassium, calcium and magnesium as well as hydroxides of sodium and potassium.

As an aside, I had never thought about the fact that in some cases the general hardness and KH or buffering capacity might be the same! Calcium carbonate would register as general hardness because of the calcium and also as KN (carbonate hardness). So, depending upon the minerals in the water DH (Degrees hardness or general hardness) may be close to the KH or they may be different. DH is often greater than KH.

There are naturally occurring waters which are alkaline but have a low overall DH. Obviously that water doesn't have a high KH, but there is enough buffering material to keep the pH up. And rainforest fish breed in it!

The natural progression of most aquariums is toward the acidic pH range. Organic decomposition usually turns our freshwater tanks more acidic/ less alkaline over time.

http://www.futurepets.com/trivia/aquariums-calcium.htm

Raising the pH and keeping it up may require increasing the dissolved mineral content (total alkalinity/buffering capacity)

As an aside, a new aquaculture business in the US is prawn growing. Prawn growers need a certain amount of alkalinity and add limestone to their ponds, but they don't add it to the water if it has a buffering capacity of over 60ppm alkalinity. If their ponds threaten to reach a pH of 9 or 10, fatal to prawns, they may dose with sugar to drop pH. The biological breakdown of that sugar will produce acids which drop the pH.
http://freshwaterprawn.org/Newsletters/spring2002.html

Ideal alkalinity for FW 120-150 PPM. That does suggest that your is a bit low. Swimming pools are supposed to be about 100 PPM. Hopefully there is little biological action there. ;)
http://www.aquatichouse.com/test%20kits.asp

In
http://www.aquarticles.com/articles/management/Van%20dijk_Buffers.html
Bart van Dijk notes that pet remedies for lowering pH include, starting with the quickest acting substances, include a spoon of baking soda (or avoiding sodium, potassium bicarbonate), garden lime, ground limestone, a small chunk of limestone, some beach sand with broken shells, some coral sand or a small piece of coral in the tank. "Each one of these, together with water and the carbon dioxide our fishes produce, will form the first buffer combination the blood uses: carbonic acid and sodium bicarbonate (H2CO3 + NaHCO3)."

I have also used tufa rock in Rift Lake cichlid tanks. The previously barren Julidochromis had fry within two weeks. Rainbowfish people sometimes use coral chips as gravel to keep the pH up in fry tanks. You might try adding a small bit of one of those suggested above in your tank. Since you are doing such a good job of monitoring your pH, rocks or gravel might be useful to you.

Even as we speak (ok, type), I've a bucket, fairly securely resting on top of a 15-gallon tank. Water is flowing through a fairly slim siphon tube into the filter outflow below. The idea is to gradually merge the waters and ease whatever adjustment is needed, if the waters are little different. That may be something you will want to do with your changing water, if you have rocks or gravel in the aquarium influencing hardness and buffering capacity.

You may decide to work up a recipe for a bucket (32 gallon garbage can, whatever) of so many teaspoons (or a tablespoon?) of cichlid salts or one of the powdered items above. That may give you water which can be more comfortably matched with your tanks. Remember though that the nitrogen cycle will still pull the pH down a bit.

Hope something in this counter-ramble is of use.