The hardness of your water would depend on the species of fish you intend to keep. African Cichlids need hard water with a very high GH/KH (total hardness) but fish from the Amazon need soft water with very low GH/KH (total hardness). I would advise you to always independently research what water parameters are required for whatever species you wish to keep.
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Yes, most freshwater fish need a air stone in their tank. And even if they don't need one it will improve the fishes health.
I filled the tank with freshwater.
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Yes but you need to buy a saltwater filter to replace the freshwater one.
no because if its a freshwater tank the rain has salt in it
yes.
waterchange
Do you have the space for them? because freshwater angelfish are generally good parents. however, you should separate the fry by gender when they grow older.
Because he is from freshwater and needs to breathe freshwater instead if air.
probably not.
The dH is the measurement of KH which should be above 4.5 dH. The KH can determine the stability of the PH in your tank. If it's too soft, you may need to add crushed coral to your filtration, too hard and you may need peat moss; this is all dependant on the type of fish you keep