Welcome back to another Fishkeeping Focus, where we'll be talking about KH in an aquarium and why it's so important to have an understanding of it's role in your systems.
KH (or carbonate hardness) is the measure of carbonates (CO3) and bicarbonates (HCO3) in your water. They are also directly related to your PH. In simple terms, high KH = high PH, and lower KH = lower PH. This knowledge is especially useful when we're looking at stocking suggestions, or if we're experience a cycle crash. 
KH's role in the aquarium as covered above, will determine your PH. Carbonate hardness is one of the most important parameters whenever you're looking into how to sustain your nitrogen cycle in the aquarium. This is because whilst your nitrogen cycle is working away, breaking down ammonia into nitrite, and nitrite into nitrate, it releases hydrogen ions. PH (Potential of Hydrogen) will lower as your cycle continues to release hydrogen. The benefit of having KH in your tank is that it will "mop up" these loose H+'s. KH acts as a "buffer" which keeps your PH stable, but as it does this job, your KH will deplete in your aquarium which will mean that you're more likely to experience a PH swing, or worst case scenario, a cycle crash. Thankfully, there are ways to prevent this! We will talk through how to measure KH, then we will talk about different ways you can either work with low KH, or how to raise it.
Typically, you will see it measured like dKH, which stands for degrees. The good part about this is it's super easy for you to measure like this! If you use a liquid testing kit like API or NT Labs, then all you need to do is follow the instructions on them, which is normally by putting 5ml of tank water in your test tubes, and add 1 drop of your KH checking solution at a time, inverting the test tube, then you keep adding 1 drop until the solution changes colour. This then tells you your dKH.
The simplest way of ensuring that your KH is within range is through water changes! By changing your water weekly, you're repleneshing your KH stocks. Water changes are an extremely important part of the process in general, as by doing so you're lowering your nitrates, and giving your tank that all-important KH that allows your ecosystem to keep on living.
If you're like us, and live in an area with very soft water (we measure 0-1 dKH in our taps!) then you'll be looking to add some form of KH raiser. There are many brands out there that can do this, and many people opt to use R/O water and replace their minerals as a standard practice as well! The brand we use to ensure our KH stays at a stable place is Aquadur. This also raises GH, but as we have said, our water is very soft so this is beneficial for us. Avoid any product that is "PH up" or "PH down" as both of these can make a cycle very unstable and are typically only a temporary fix. Now that you've mastered the importance of KH in the aquarium, you'll never be fooled by these products again!
The last way of dealing with KH is of course, working with what you have from your taps! The greatest thing about fishkeeping is that there is a whole range of fish out there and they all love different parameters. So if you have a lower KH than normal, there is always going to be a fish out there that will feel best in those parameters. If you have high KH, you can always look up some fish that prefer a higher PH. 
We hope this helps you understand a little bit more of what happens in your tanks, and how you can work with the knowledge you have to keep things running smoothly. As always, if you're struggling to understand or need a little bit more advice on the matter, we at HFK are more than happy to help.
