Not trying to confuse you, my friend, but these days the pH and most of the water parameter stuff is just not an issue. The vast majority of fish you will get a your local fish store are hatchery raised, predominantly in higher pH and harder water conditions. I keep altum angels, for example, in very hard, high pH Houston TX tap water with no issues whatsoever-- and these are wild caught! You can see a clear image of these fish on my website under the "Hardscape Only" gallery. Unless you are trying to breed fish or induce some very specific behaviors, this whole pH matching business only confuses and deters new hobbyists unnecessarily.
Why can I say this with profound certainty? Because it's my life- my career- my absolute passion and I have an unmatched portfolio to back it up. We have over 500 fully aquascaped, freshwater aquariums of every imaginable size and makeup in our care and we do not test water and do not concern ourselves with matching "parameters" for the vast majority of fishes- right up to cardinal and rummy-nose tetras, discus, and many other notoriously "sensitive" fish. I only point this out to offer a powerful visual reference to substantiate what I am saying, not to brag. The Internet doesn't need any more "self-styled experts", and that's all I'd be without proof.
I'd rather see you be able to focus on creating and enjoyable, engaging aquascape rather than fretting over what pH your tap water is. I'm telling you in more than most cases the common fish you will be keeping will and/or already are perfectly adapted to your tap condition.
In a tank your size, you may want to consider more of a themed tank- not trying to squeeze too many different species, but rather a feature fish, and accompanying/complimentary school around them, and a bottom dweller group to complete the scene. Maybe think about the creation of an underwater scene- an aquascaped- and not just a "fishy tank". Tetras, many small barbs, danios, rasbora, cory cats, dwarf loaches, dwarf rainbows- all pose viable, interesting options for a 20 gal. I mean, if you want, I (or many here) could spec some specific combinations for you, but you probably get the idea!
I will defend but not argue about the above comments. Ive been doing this too long and at too high a level to argue about realities that I experience and deliver on for hundreds of clients everyday. It pains me to see so many new hobbysist frustrated and turned off to the hobby because of being overwhelmed with factoids and details that are not "incorrect", but often unnecessary. :-D