This page describes proper water quality for your fish and things to consider and bear in mind when caring for goldfish.
Purchase water test kits from the pet store. You'll want to get measurements for 5 things in your water before you purchase a tank and go to the trouble of setting it all up.
- Your water's pH should be between 7.5 to 8.5. Any more or less and the goldfish may have decreased longevity, but bear in mind that wild swings in pH are deadly so getting them used to your tap water and testing it often (perfect would be at every water change, but I test mine every 6 months) is more desirable.
- The Carbonate Hardness (KH) should generally be at least 100ppm or better. Most people have softened water in their home so this can often be difficult. There are hardness buffers that you can purchase and other things you can do, like adding crushed up oyster shells to your aquarium to increase the hardness. pH is more likely to be stable and less likely to swing drastically if there is a medium to high hardness level.
- Ammonia should be less than 5 ppm, the lower the better. Ammonia is a byproduct of fish waste and as it builds, the biological filter in the tank is supposed to convert this into nitrites. Ammonia is poisonous to fish. As dirty as goldfish are, ammonia, can build up very quickly. When first setting up a tank, it's important to monitor this every day until it is stable and low for a month. You don't want any ammonia in the tank at all.
- Nitrite should be as low as possible as well. <5ppm. Nitrites are created by bacteria in the tank converting fish waste into it from the Ammonia. Once this is done, another bacteria converts the nitrite into nitrate.
- Nitrate should also be as low as possible and definitely lower than 40ppm Nitrates are the end product of an established bacterial filter but it will definitely build up and this is why partial or massive water changes (depending on the level) are required. It is important to note, though, that while Nitrate is bad for your goldfish, it's not nearly as toxic as Ammonia or Nitrite. It should also be noted that some plants will help to absorb Nitrate in the water but they cannot keep up with even a single goldfish.