Fishkeeping is actually waterkeeping. If you keep the water well, your fish will be well as well!
About Water Parameters
We have been drilled in science class to know that water is H2O. This gives us the impression that water in nature is just that. However, it is not. Water in nature is very rarely pure. It contains 'impurities' such as salts, minerals, nutrients etc. Since the environment of the water bodies are different in different places, aquatic life in different water bodies have adapted to water with different 'impurities' content.
The easiest fish to keep is those that have adapted to water qualities that is similar to your local tap water so you don't need to adjust your water content to match that of the fish's natural habitat.
1. If you want to replicate the conditions of your tank water so that the water in your tank is the same as those your fish lives in nature, you will be looking at:
pH, salinity, buffering capacityand general
Read about them here
However, generally fish are adaptable so actually as long as you don't change these chemical composition suddenly, everything will be just fine. Of course, if you want them to be at their best condition, then you can SLOWLY adjust these variables to their best range for different species of fish. It is however best that you don't itchy backside because these parameters affect one another. When you change one, it changes something else as well. In fact, these parameters don't only change themselves, they sometimes affects the chemical gangsters mentioned below too! (for example, some toxic ammonia is formed from harmless ammonia when pH is increased!)
2. When things go wrong in your aquarium, 99% of the time it is not due to those parameters mentioned above such as pH and hardness, but the 3 notorious chemical gangsters which are:
ammonia, nitrite and nitrate
Read about them here
These 3 chemicals are harmful to fish and you want to have zero of them if possible, and if not zero as little as possible. These are so harmful simply because fish CANNOT adapt to them. It's like humans can adapt to living in different temperatures and humidity - we feel uncomfortable at first but we get used to it; BUT WE CANNOT adapt to toxic gas! For this reason, if you find a magic teapot and you are given the choice to learn one thing, choose to learn about these gangsters!
If your testkit reading is always at the highest reading, don't assume that your water is at the highest reading. It could be higher than that but the results cannot be reflected due to the testkit's limitation. This is a way to overcome the testkit limitation.
What is pH?
pH is what tells us whether the water is acid or alkaline.
pH below 7 is acidic
pH 7 is neutral <--- middle ground where it is neither basic nor alkaline
pH above 7 is alkaline
Congratulations! That's all you need to know
Most fish can adjust themselves to water of different pH as long as it is not too much. If your tap water is alround 7 it would be suitable for most fish.
It is better to keep your pH stable than to adjust it all the time because it is stressful (not stressful for you.. stressful for the fish!). You need to monintor your aquarium's pH so that you know when your aquarium pH crashes or shoots up. Most of the time, it is sudden change in pH that kills fish and not because the pH has gone out of the "fish's pH range".
One thing to note is that when pH rises, this change can impact on your water parameters. For example, when pH rises harmless ammonia changes to toxic ammonia! (another fish killer).
Salinity refers to the salt content of the water. Basically, the main thing you need to know is whether the fish is saltwater (marine) fish or freshwater fish.
You can measure salinity using a hydrometer. Only marine fish keepers need to measure salinity. Freshwater fishkeepers should only add a small amount of salt to your aquarium. Why you should add salt to your aquarium
One thing to note is that fish without scales are more sensitive to salt than those with scales.
Buffering Capacity / Carbonate Hardness (KH)
Buffering capacity refers to water's ability to keep the pH stable. Think of it as a sponge. When you pour acid into the tank, it absorbs and neutralise the acid and so your pH only drop a little. However when it absorbs the limit it can hold, it's 'buffering capacity is used up'. When this happens, when more acid is added the pH drops drastically. This 'sponge' is actually carbonates. This is also why buffering capacity is sometimes called 'carbonate hardness'.
One trap beginners often fall into is when they try to lower pH of their water. Some tap water have a high buffering capacity. When they try to lower the pH they find that there is minimal effect when they add a lot of chemical adjusters such as pH down. As a result they confidently add more and more. However, once the buffering capacity is full, the pH suddenly crash with all that acid and all their fish dies.
Bufferning Capacity is important because without enough buffer, pH drops during aquarium cycling and when nitrates(end result of aquarium cycling) is produced. This pH drop can be buffered so that the pH stays at a stable level.
Buffering Capacity (KH) does not affect fish directly, so there is no optinum level for fish. The higher KH, the more stable the pH and vice versa.
General Hardness (GH)
Now general hardness sounds like the same thing as carbonate hardness but its a different thing so don't mix up! General Hardness or GH is the measure of calcium and magnesium ions in water.
General hardness is what people means when they say this fish lives in soft water or hard water.
You can see general hardness measured in dH or ppm (part per million of CACO3)
1dH = 17.8 ppm
Ammonia is poisonous to to fish. If you ever heard people who say "I can't keep fish they all die on me fast", it is not because they don't have a loving heart. It is because they don't know ammonia.
Ammonia is formed quickly from fish waste, rotting food, or dying plants. In fact, anything that causes a bad smell shows the presence of ammonia. The bad smell is not shit! It is ammonia! (You might want to know stink bombs kids throw in class are made of ammonia too)
Ammonia attacks the fish gills (their delicate breathing organs) and fins. If you see your fish panting for air, don't immediately think you need to upgrade your air pump. Check the ammonia level!
Nitrite is less harmful than ammonia, but he's not your friend either. It just means fish can tolerate them better. They are still toxic at higher concentrations!
Nitrate is the least toxic of the trio. It doesn't burn the fish directly but high nitrate levels lowers the fish resistance to diseases (such as the common white spot) and fungus and bacteria attacks.
Do you know if you eat consume too much food or water that contains high nitrates it is harmful to humans too? In fact, nitrates in well water can also cause irreversible damage to your nervous system and cancer in children and infants. (Brough, Holly B., and Durning, Allan B., "Taking Stock: Animal Farming and the Environment," Worldwatch Paper 103, by Worldwatch Institute, July 1991, p.19-20)
Ammonia, nitrite and nitrate actually have a close relationship. Please read aquarium cycling to see how they converts to one another and how to neutralise them naturally.
The end product of cycling is nitrates. This nitrates will increase over time unless you have plants or a denitrator to consume the nitrates. Water changes does not control nitrates.