In our tanks what holds pH from dropping is KH.
Buffering Capacity (KH, Alkalinity)
Buffering capacity refers to water's ability to keep the pH stable as acids or bases are added. pH and buffering capacity are intertwined with one another; although one might think that adding equal volumes of an acid and neutral water would result in a pH halfway in between, this rarely happens in practice. If the water has sufficient buffering capacity, the buffering capacity can absorb and neutralize the added acid without significantly changing the pH. Conceptually, a buffer acts somewhat like a large sponge. As more acid is added, the ``sponge'' absorbs the acid without changing the pH much. The ``sponge's'' capacity is limited however; once the buffering capacity is used up, the pH changes more rapidly as acids are added.
Buffering has both positive and negative consequences. On the plus side, the nitrogen cycle produces nitric acid (nitrate). Without buffering, your tank's pH would drop over time (a bad thing). With sufficient buffering, the pH stays stable (a good thing). On the negative side, hard tap water often almost always has a large buffering capacity. If the pH of the water is too high for your fish, the buffering capacity makes it difficult to lower the pH to a more appropriate value. Naive attempts to change the pH of water usually fail because buffering effects are ignored.
In freshwater aquariums, most of water's buffering capacity is due to carbonates and bicarbonates. Thus, the terms ``carbonate hardness'' (KH), ``alkalinity'' and ``buffering capacity'' are used interchangeably. Although technically not the same things, they are equivalent in practice in the context of fishkeeping. Note: the term ``alkalinity'' should not be confused with the term ``alkaline''. Alkalinity refers to buffering, while alkaline refers to a solution that is a base (i.e., pH > 7).
As for the out-gassing of CO2. I have well water. When I set up my first tank in Jan. 2001 my testing said my water was acid pH when I tested it from the tap. But I did not know about it being possible to have high levels of CO2 in the water that came from the tap. Testing in the tank showed it to be about 7.4. And then I was educated on out-gassing and learned my tap water really was 7.4. It takes very little time for tap water in a small container that is being aerated to bring the water to its natural CO2 and O levels. All this takes is surface agitation. This is also usually how we get gasses into and out of our tank water. To do the out-gassing for texting pH it takes a matter of minutes for the CO2 level to hit equilibrium either out or in-gassing depending n the state of the tap water.
Next, the nirtifying bacteria in our tanks as well as some of the live plants can get the carbon they need from carbonates/bicarbonates as well as CO@. So, over time and in the absence of anything replacing them, carbonates/bicarbonates can get used up. And as they do the pH will drop.
For years I had a tank with Altum angels in which I used RO/DI water. I mixed it with my tap and never needed to mineralize it at all, In fact at times I had to add a bit of muriatic acid as the pH drifted up. The tanks was targeted to be int eh 50s ppm-wise for TDS and the pH at 6.0 and never higger than about 6.4. I kept a pair of discus for a few years in my tap with no issues.
What I have learned is that there is a difference between wild and tank raised fish. This is especuially true with species which have been regularly line bred for color and/or for fins. Angels and discus are perfect examples. When I was starting ith Altums it was with wilds and the tank for reeivng themn was 20 ppm TDS and 4.2 pH. In the end the discus I kept ofr the longest we tank bred in Las Vegas (very few people can do this). They loved int he 6.0 50-60ppm TDS water for years until I sold them a few months back.
In reading this thread where it starts with the pH being off between the tank and tap and then no change in the bucket and then the pH from the tap being 7.4 in the bucket, I came to the conclusion that the most likely explanation could be testing error. I could be wrong in this but JIK, here are my rules for how I do it.
- Rinse the test tube in tap water 1st and shake it somewhat dry.
- Dip the tube into tank several inches below the surface to fill it. Then empty it pouring it into the cap so that rinses as well.
- Hold the tube open end down, submerge it to mid-level of the tank and fill it. Don't take water from the surface anything floating there goes in the tube.
- Perform the Test. Make sure to use the cap and never a finger to seal the top of the tube when shaking/inverting to mix.
- Dump the solution out of the tube and then rinse the tube and cap out with tap.
For my Altum tank I had a continuous digital monitor which measured conductivity or TDS, temperature in F or C and pH. I transferred the probes to the can of changing water so the parameters there would bring my tank back closer to the target levels. I could not use the liquid test as I had very stained water using almomd leaves, alder cones and Rooibos tea.
neutrino
I actually came back to the bucket test again yesterday. Giving it another go. Yes there is a single air stone in there. Last time I only did it for 24 hours. Since the tank seems to change after about 72 hours I’m gonna see what it reads up to that. 
duanes
does it? Sometimes it’s hard for me to tell. I know we’re not talking about a huge swing (0.6 if my numbers are correct) The fish seem to be fine. I just worry about the long term effects of that. I always complicate things. lol. These fish are just so god damn beautiful. I had to do it. 
