An amazon sword would completely outgrow a 20L in about a year. It is surprising a standard T8 20L hood is doing all that for you.
You could comfortably put a group of celestial pearl danios in the 10 with ADFs and mystery snails.
I wouldn't be concerned with Ph, GH and KH at this point. The elephant in the room is your nitrites. They are more lethal than ammonia and you need
Ha, oops, meant to say welcome to AC. :p
All babies still ok swimming around & catfish ok. Haven't put any food in tank for 4 days.