It is never a good idea to use chemicals to change the pH.
You know your father's address, could you not check on his water provider's website for his hardness?
A list of fish that can go in a 20 long
Brisle nose pleco
Dwarf gouramis (my personal favourite)
Siamese algae eaters
Chinese algae eaters (aggresive)
I am sorry but this is a terrible list.
Neons - OK if the hardness is soft
Cories - the dwarf species are OK if the water is soft; the larger species need a bigger tank
Bristlenose - poops so much a 20 gallon would get messed up quickly, and really needs a bigger tank
Zebra danios - need a bigger tank
Goldfish - cold water fish which need a bigger tank
Dwarf gourami - OK size wise but most are infected with incurable diseases by the time they reach the store
Siamese algae eaters - need a much bigger tank
Otos - not recommended for new tanks as there won't be enough algae to feed them
Guppies - OK if the water is hard
Chinese algae eaters - need a much larger tank and are prone to eating the slime coat off other fish
German rams - need very soft acidic water and a higher temperature than most other fish. Could do with a bigger tank as well.
Endlers - OK if the water is hard.