Hi, There a house where the GH in the water thats coming out of the tap is really high, over 21D . I wanted to ask what effect can it creat on the fish? and do you know of ways to solve it? is there some kind of tap filter that can be used to make the water better for the fish?
The effect of high gH depends on what fish are kept in the water. Those that like hard water such as Malawi and Tanganyikan cichlids, livebearers like mollies, guppies, platies, and swordtails will all be fine. Fish that prefer soft and acidic water such as most tetras, rasboras, South American cichlids etc will suffer long term damage to their liver and kidneys and osmotic stress suppressing the immune system and leaving them susceptible to disease.
The only way to lower gH successfully is by removing some of the mineral content. This is best done by using a Reverse Osmosis filter which strips all solids out of the water. The correct amount of minerals for the fish being kept then needs to be added back in, either as powdered commercial preparations or by mixing the RO with a proportion of ordinary tap water to get the desired gH and kH. There's an article here on the use of RO water: https://www.fishkeeping.co.uk/articles_109/ro-freshwater.htm