Yup, that's why your pH keeps bouncing like this.
Can you get your hands on crushed coral or dolomite - That will up your hardness and in doing so will provide a MUCH more stable pH.
Now the problem is: The water your tap has is PERFECT for you Neon's - Just not for any other fish....So if I was in your shoes (and that's obviously up to you) I'd set up a 2nd tank simply with tap water for the Neon's.
Then for all other guys up the hardness in the tank and they will be happy.
In general: I'd personally add plants as they'll help a stable eco -system in the tank (so less headache for you and no need for all these various chems you had)
If you chose to ulter your water: Please do not use chemicals they'll make it drop & rise too fast (and you already seen where this leads to). Adding types of rocks that will up your KH and so with stabilize your pH is much safer (and I'm sure Byron will comment on this as well, he's went by this method for many yrs).
I suspected it was your KH yesterday, which really is good in a way...Cause if you'd have come back now and told us your KH was like 6-7 then I'd need to get digging in the books here about a mystery lol
You already won the battle half way now, knowing what's going on and that you have fish that have entirly different needs. Now you can get to work, or also alternativly exchange the fish for others that will thrive in your water