How low a ph b4 a crash

[swifto]

As the title says, I've always have used a API freshwater marster test kit.
My ph has always been 6.0 according to the API test kit as this is the lowest it goes,today I've tested the ph using a digital ph pen and got a reading of 4.9.
I don't use anything to lower it,my tank contents r,seachem flourite sub straight (56kg),8.5kg of aqua soil,amazon swords a large piece of d/w from illusion.Fish are 6 discus 12cm+,1b/n,1 cory and a clown loach.
I've not checked the gh,kh at all.So how low is to low,guess I've got to buffer.any advice

[rex82]

Ph crash an having a low ph are 2 different things, IME once it hits around 4 it won't fall anymore. A ph crash is when the ph suddenly drops, this will definitely harm the fish.

[rex82]

Having no kh is the cause for sudden drops.

[Merrilyn]

Swifto, get a bag of coral fragments from your LFS and keep it in your filter to buffer the water. It will raise your pH gently over a matter of weeks. Never make drastic changes to your pH, although a sudden drop is more dangerous than an increase.

[swifto]

Thanks for the advice,I thought of coral pieces but was concerned but the calcium increase from the coral.I added 1 teaspoon of sodium bicarbonate last night and this morning ph is 6.2,so coral is a more natural solution i don't like to chemicals in may tanks to buffer the ph.
Thanks