What fish do you keep?
What is the pH in numbers?
Take a sample of tap water and check the pH. Let the sample stand for a couple of days and check the pH again and see if it has dropped.
What is the GH (general hardness) and KH (carbonate hardness) of your water supply?
This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
Depending on what the GH of your water is, will determine what fish you should keep.
Angelfish, discus, most tetras, most barbs, Bettas, gouramis, rasbora, Corydoras and small species of suckermouth catfish all occur in soft water (GH below 150ppm) and a pH below 7.0.
Livebearers (guppies, platies, swordtails, mollies), rainbowfish and goldfish occur in medium hard water with a GH around 200-250ppm and a pH above 7.0.
If you have very hard water (GH above 300ppm) then look at African Rift Lake cichlids, or use distilled or reverse osmosis water to reduce the GH and keep fishes from softer water.
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Most water companies add pH buffers to reduce corrosion in water pipes. Chlorine also tends to raise the pH.
You can use a reverse osmosis (R/O) unit to make close to pure water and then mix the R/O water with some tap water to get a better pH.
You can buy bottled or distilled water and mix it with tap water but that is expensive.
You can make a solar still and mix the distilled water with tap water. It is slower but free once you have the components.
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SOLAR STILL
You can make a solar still. It would give you pure water, no waste water and be free to make pure water, it just requires a bit of sunlight.
Get a large plastic storage container and put it outside in the sun.
Pour a bucket of water into the storage container.
Put a clean bucket in the middle of the storage container. Have a rock in the bucket to stop it floating around.
Put the lid on the storage container.
Put a rock or small weight on the lid in the middle, so the lid sags above the bucket.
As the sun heats up the container, water will evaporate and condense on the underside of the lid. The water will run towards the centre and drip into the bucket. When the bucket is full of water, you put it into a holding container and put the bucket back in the storage container with another bucket of tap water.
You get pure water with a pH of 7.0, 0 GH, 0KH and no wasted water, no power used and it's cheap to set up. You can then use this water to mix with tap water and get a better pH, GH & KH.