What everyone is talking about is hardness, also called GH - the amount of calcium and magnesium in the water. Rivers and lakes can have a lot of calcium & magnesium, and this is hard water. others have only a small amount of calcium & magnesium, and this is soft water. Fish can come from either hard or soft water and their bodies have evolved to cope with that type.
Fish from hard water have evolved so they excrete most of the minerals they take in from the water. if they are kept in soft water they continue to excrete the minerals but there aren't enough in soft water to replace them so they suffer calcium deficiency and get sick more easily.
Fish from soft water have evolved to hang on to the few minerals in soft water. If they are kept in hard water, they still hang onto minerals - too many of them and they get calcium deposits in their kidneys.
We should aim to keep fish which come from water similar to our tap water. Since hardness doesn't change in a fish tank unless you deliberately do something to change it, you don't need to buy a GH (hardness) tester. As mcordelia suggested, take sample of your tap water to an LFS and ask them to test it for GH - and make sure they give you a number.
Do you have your well water tested to make sure it's OK to drink? That report might contain your hardness.
Of the fish in your list:
-4 guppies - hard water
-10 dwarf neon rainbows - soft to hard
-4 pearl gourami - soft ( and 1 male, 3 female)
-2 dwarf petricola - hard (and a group of at least 4)
-4 black or neon tetras - soft (and you need at least 6 with more being better)
-5 panda corys - soft
You will need a test kits which tests for ammonia, nitrite, nitrate and pH. They are necessary for cycling the tank before you get fish.
Cycling Your First Fresh Water Tank What is Cycling and Why is it Important? Fish waste, and especially fish breathing, plus uneaten food and other organic matter breaking down in a tank all produce Ammonia. This can quickly become toxic to fish if it is allowed to build up to any measurable...
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