PH page in progress:
Ph (Potential Hydrogen) a measurement of hydrogen Ions in the water.
KH (Carbonate Hardness) is a buffer of your ph.
GH (General Hardness) a measurement of minerals in your water.
Ph is natural at 7.0 anything above that is Alkaline (base) and anything lower than that is Acidic. Goldfish like 7.0 to 7.5 anything lower than 7.0 can cause problems with goldfish.
More minerals and carbonates will increase the ph levels. If you are cycling the tank or have high amounts of ammonia or nitrites in the tank water this will lower the ph level as it uses up the minerals and carbonates in the water. Also organic matter as decaying wood will bind the minerals and carbonates resulting in lowering the ph levels.
Other things to think about when the ph starts to drop down is over stocking and plants. Plants in the day time will produce O2 cause they use Co2 for food this causes the ph levels to go up. Now if you only have one or 5 plants in the tank of lets say a 20-30 gal tank this wont effect the ph levels that much. If you over stock your tank it will lower the ph down to because goldfish produce Co2 all day and night. One good reason not to over stock your tank (one goldfish per 15-20 gallons) or you can produce to much Co2 in the tank. If you don't have enough agitation at the surface of the tank this can cause the ph to lower to cause of the extra Co2 in the tanks water due to the fish.
Now lets say your kh has gotten to low and your ph is starting to drop on you. You could use crushed or slabs of limestone, crushed coral or seashells from your LFS. Now u could either use them as gravel in the tank or use netting and place the crushed limestone, coral and seashells in a filter so that it will be washed into the tank as the water flows through them. When these start to break down they will release minerals such as calcium, magnesium and carbonates this helps keep the KH to be more stable if your KH is falling if the KH is falling it can make the Ph fall. If you get your KH to 100 or above this will stable your ph but if your KH gets above the 300 mark you can hurt your fish's gills. If your KH out of the tap is low you will need to have a buffer to keep it up or the ph will fall with it.
If your ph drops to low you can kill your fish, also you should always check w the LFS were your buying your fish from so you know what the fish's ph is in now compared to yours. If its .5 off either higher or lower u will need to do a slow introduction to your water or the fish can go into ph shock. One way to slowly help them into ur ph if its different is a drip method. This includes a bucket with a rubber airline and a line clamp that controls the flow of the water coming out of the tank, having it drip one or 2 drips a second. This will let the fish slowly get use to ur ph level. The other way if you don't have this is to float the bag in the tank, then every 5 mins let a 1/4 cup of your water into the bag, doing this over again for an hour, then release the fish into your tank. Keep a close eye on the fish for the next couple of days to make sure it doesn't go into ph shock.
So if your ph is dropping and your kh isnt above the 100ppm mark you need to get a buffer, make sure you have plenty of water agitation to help to release the Co2 to help the ph raise in the water.
If you have the GH and KH tests and want to know how much minerals are in the tank you can test both of them, then subtract kh from gh this will show you the levels of minerals in the tank.