Welcome! That's a cool little community tank you have there. I keep ADF's myself, the only thing I would say is that 75g is probably too deep for them, they're not great swimmers. Personally I would keep them in a smaller tank either to themselves or with something peaceful like pygmy cories. That way you can target feed them and ensure you're not overfeeding the tank. Those pellets are probably sinking down in between the gravel and decaying in there.
Also I wouldn't worry about adjusting your pH unless you want to start adding RO water and remineralising it. Your fish can get shocked from sudden pH changes (trust me, I know from experience - lost all my fish and only my ADF's survived) so I'd just work with what you have naturally.