Hi and welcome to the forum
If you ever get an ammonia or nitrite reading above 0ppm, or a nitrate reading above 20ppm, do a 75% water change and gravel clean the substrate every day until the levels are 0ppm.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
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Some pet shops will buy/ trade fish from customers. The fish have to be similar size to the ones they sell in the shop, and must be healthy and free of disease. Contact the shop beforehand and ask if they buy/ trade fish from customers. Let them know how many, colour, size and see what they say.
Some people are selling fish on Ebay, etc. If you do this, specify local pick up only.
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What fish do you have in the tank?
Before adjusting the pH, what is the pH, GH and KH of your tank water and the tap water?
When you test the pH of the tap water, test it straight out of the tap, and then 24 hours later. See if there's a difference.
If you don't have a test kit, 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).
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You can raise the pH of water by adding limestone, sand stone, shells, dead coral rubble/ skeletons to the tank or filter. You only add a small amount and monitor the pH over a week. If it hasn't gone up, then add a bit more and monitor over a week. Continue adding them until you get the right pH.
Shells, limestone, coral rubble, etc are calcium carbonate and can raise the pH to 8.5.
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If you have soft water with a GH below 250ppm, you can buy Rift Lake water conditioner form pet shops. These are made from calcium and magnesium chloride and carbonates/ bicarbonates, and can be used to increase the pH, GH and KH.
Riift Lake conditioner needs to be added to new water and aerated for 24 hours before that water is added to the tank. You add the Rift Lake conditioner to all the new water that goes into the livebearer (molly, swordtail, platy, guppy) tanks.
Mollies need a GH of at least 250ppm and a pH above 7.0.
Guppies, Platies & Swordtails need a GH above 200ppm and a pH above 7.0.
