Many ponds and new ponds hobbyists aren’t aware of GH & KH (or water hardness to put it simply), and as a result don’t understand the vital role it plays in fish health. GH or ‘General Hardness’ refers to the dissolved levels of magnesium and calcium ions in the water, while KH or ‘Carbonate Hardness’ refers to the measure of carbonates and bicarbonates dissolved in the water.
It is important to have a good KH in your pond or aquarium in order to keep pH steady and stable for your fish. The KH has a buffering capacity to neutralise nitric acid (produced from the nitrogen cycle). Without KH the water would become naturally acidic over time which could result in fish fatalities. In areas of the country with hard water, the buffering will be a natural occurrence. As mentioned earlier, areas of the country (such as Manchester) with soft water, will normally need to introduce KH to their pond or aquarium in order to keep pH consistent.
Naturally occurring water in the form of streams, rivers, ponds, lakes etc. is rarely purified in the distilled water sense. Instead, it is packed full of dissolved salts, buffers and nutrients, and the levels of these will vary according to geography and local conditions. Fish of all types worldwide, have evolved over millions of years to inhabit very specific water conditions, and it is our job as pond/aquarium keepers to try and replicate these conditions in order to keep the fish happy and healthy.