First things first. One of the most important factors of keeping fish is cycling their tank. This process can take up to two months to complete. To further understand the Nitrogen Cycle, we will refer to the visual above. Goldfish produce a toxic chemical through their skin and waste called Ammonia. Several things can increase and decrease the level of this chemical in your tank. In natural streams and ponds, the ammonia is eaten away by bacteria, which live in the gravel and surfaces of the area.
The bactaria, in turn give off Nitrites which is also a dangerous chemical if not controled and if let loose will poison your fish to the point of death. The Nitrites are then turned into Nitrates through another kind of bacteria and then the Nitrates are used by plants, and eaten by algae. Water changes throught the cycle are needed at different rates and sizes. The general rule is to rotate or displace 25% of your tanks water with fresh, clean, dechlorinized water.
So if you have a 20 g tank you want to take out 5 g and put in 5 g. The frequency of this will vary dramatically depending on where you are in your cycle. If you set up the tank five days ago, you'll change your water nearly every other day...But if you set up your tank five months ago you will only need to change the water weekly. This depends on two major things, one being the ammount of beneficial bacteria you have in your tank, and the levels of chemicals you have in your tank. But how do I know how much Ammonia and Nitrite I have in my tank? The answer is as simple as the question, you have two choices, you can take your water to a local pet shop or store and have them test it or get a test kit of your own and test your levels at home.
If going to a pet shop every week at least is too much of a hastle, get your own test kit. There is also another chemical we haven't discussed yet. It is pH which refers to the hardness of your water. Hardness of water depends on the mineral content. Ocean and Sea water has tons of salt in it, making it hard water, but pure water, say drinking water, is more likely to have a soft hardness. pH is refered to as either Alkaline or Acidic water and is measured in ppm or parts per million. When you test or get your water tested, you are hoping for a 7.0 reading however, around there is fine.
To maintain goldfish, you want a steady level of pH. The more your pH fluctuates the more stress there is on your fish. You shouldn't worry too much about pH unless it's way off, like a 9 or something. But as long as it's it's within the high 6's and low 8's don't worry too much... So now that you have read all about the Nitrogen cycle, and why it's important to change your water, as well as what pH is, how it's measured, and how it affects your fish.