what moves ph?

[Spewn]

With a high Kh value pH drop is resisted through chemical buffering(reactions). The calcium carbonate(CaCO3) dissociates in the water giving you Ca+ and CO3-. An acid is, essentially, free H+ ions in water. A base is essentially free OH- in water. In order for the pH to go up or down, the concentration of H+(or OH-) must change. Increase H+, pH goes down(becomes more acidic).

All of these things reach an equilibrium within your aquarium, and the more CaCO3 you have in the water the harder it is to unbalance that equilibrium. If you add H+, you'll get HCO3, add OH- and you'll get Ca(OH)2. The more CaCO3 there is in your water, the more this will happen instead of the pH changing.

Edit; Didn't see karl's last post, but yes, it doesn't have to be Calcium carbonate. As you can see from how they react, the CO3(carbonate) portion gives the negative charge, other positive ions could take calcium's place with a similar(same) effect.

 

[ksane]

Wow-thank you! That's the best and most detailed explaination I've ever seen. I, for one, appreciate it very much.

 

[Opicana]

If you KH is low (like 2), should you add something to help buff the water? I know adding chemicals to the tank other than prime is bad, but are pH swings worse?

 

[Spewn]

pH swings are probably worse than having too much(Wrong wording, should read more like "more than you need") carbonate in the water. Unless you have a species in your tank which you know for a fact does not tolerate very hard water*!*, you're not likely to cause problems increasing your kH.

The most commonly-recommended way, that I've seen, is to get a filter media bag from the store and fill it with aragonite(you can use crushed coral as well). Place this bag either in your filter, or in some high-flow area of the tank. The CaCO3 will dissolve at a rate hopefully fast enough to stop your pH from having any kiniption fits, but not so fast that you should ever really need to "replace" it. Maybe rinse it off, depending on how it looks.

Adding dry chemical to your aquarium is a pain because it needs to be re-dosed at every water change. This wouldn't be so bad if water didn't evaporate from the aquarium, but it does, so I would recommend against altering the water chemistry directly.

*!*NB: Don't use online references or books as a determination for whether or not you have a hard water sensitive fish; Reference material generally refers to wild-caught species, whereas *most* of what you see in the store is tank-raised(not all, ask the lfs and others for more info). Use either personal experience or the experience of others with that fish.