Ph levels for tropical aquarium?

Leave the pH alone unless you have a lot of experience. A lot of the products don't work, or they don't seem to work until suddenly your pH has gone up or down two points and all your fish are dead. Fish cannot handle pH swings. It's a lot of work to manipulate freshwater pH with chemicals. The animals that you list, including the frog, are very likely produced by breeders and are very adaptable to normal pHs. Your pH is fine for a frog. Most gouramis and tetras like softer, more acidic water. Livebearers like the mollies and platies like hard, basic water. So you couldn't adjust it to all of their natural pH anyway. You can ignore this, but rainbow sharks get way too big for a 20 gallon, in my opinion.