You can buy pH tester strips from any pet shop, use them to determine the current pH. level, which is how acidic or basic the water is, and then you can buy tablets to correct the pH and alkalinity to a neutral 7.0. The brand of dip test strips I use is Jungle Quick Dips and the stabilizer I use is Jungle Correct pH, they have worked thus far since I have became an aquarium hobbyist.
I filled the tank with freshwater.
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
There are no shortcuts. Get your Alkalinity right, then the PH. If the PH is "bouncing," your choline will spike high and low until you get the PH right. The PH will not be right until the alkalinity is right. If all that is right, you are low on stabilizer.
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.
Because he is from freshwater and needs to breathe freshwater instead if air.
probably not.
10 minutes
no because if you do it will die.
It is not harmful. But if you want to clean up the algae in the tank you want to get a pleco. They are very peaceful fish and will not harm angelfish. I have a pleco and three angelfish in my 35 gallon tank right now and they get along great!