Don't care about PH: Endlers, Spike-tailed Paradise fish (make sure it's P.dayi, cause the other one under this common name reaches 3 inches), captive bred Microrasboras or Boraras rasboras, bumblebee goby, dwarf puffers, least killies, some pygmy sunfish.
Rhinogobius the smaller one. There might be a couple I missed.
Softer water: Gertrude's Rainbow, Licorice Gourami, Sparkling gourami ( I think as long as you're by neutral you'll be fine) a couple other betta species. Wouldn't risk putting a vallanti in there if something is jumping into the water. Scarlet badis.
Rhinogobius the smaller one. There might be a couple I missed.
Softer water: Gertrude's Rainbow, Licorice Gourami, Sparkling gourami ( I think as long as you're by neutral you'll be fine) a couple other betta species. Wouldn't risk putting a vallanti in there if something is jumping into the water. Scarlet badis.