Your tap water has a pH of 6? That's too low for Africans.
I would switch to things that appreciate a lower pH. Angels can tolerate your aquarium's pH of 6.5ish very happily. Apistogramma would be another good choice, or blue rams. And maybe some tetras like cardinals.
To raise your pH the safest thing would be to use a buffer in a jar that's properly measured and stable. You can use seachem's Malawi/Victoria buffer. Easily available online. Mix it in a bucket until you get the desired pH and hardness and then add it to the aquarium. Don't change the pH all at once. Change 5 or so % of the water at a time and add in this buffered water.
You have bigger problems than your low pH though. The ammonia, being that high, is totally lethal to your fish. You should get something like seachem prime or Ammo-Lock and try to detoxify the ammonia you've got. Honestly, I'd remove the fish and take them back to the store if I were in your position, get your tank under control, and then add fish back slowly.