Ok, so first of all. We have some major issues here.
The tank is way too small for any of those fish to survive long term as they will all outgrow it.
The pH swing is causing them stress which is then effecting their immune system and has lead to the ich outbreak more than likely.
To cure ich without using meds, it is very important to slowly raise the temperature to above 80F, The lifecycle of the ich parasite will be sped up with the higher temperature.
You are going to have to perform DAILY without exception water changes and vacuum the gravel otherwise the ich will keep occurring and infect the other fish if it hasnt already.
Neon Tetra like to be in shoals of 5 or more so they feel secure, it can stress them if there are less.
Guppies (do you know the sex of the ones you have?) can be quite aggressive if you have male and females..either a male to a few female is best or all of one sex.
I am sorry for the information and it is probably not what you want to hear but changes are going to have to be made.