Sudden pH drop with Fish in tank!

[minyip]

Hello. I've recently started my first tank. I'm new to fishkeeping and mistakenly thought my tank was cycled when I got it because it was second-hand and had used filter media and substrate. That was my first mistake. I went out and bought some Rummy-nose tetras as the guy at the fish store told me they were good for beginners. Second mistake.
So I've been in the process of doing an emergency fish-in cycle. I've been using Prime and bacteria along with regular water changes and the ammonia levels were almost getting back to zero. I was hopeful thinking my fish had almost made it through unharmed.
Then yesterday I checked on my fish and they were all sitting at the bottom and one was dead. I did all the tests and ammonia, nitrite, nitrate all seemed normal, but the pH had dropped to 6 or below.
Normally the tank stays around 7.4. I've done some research and the only cause of pH drop that I can find that would relate to my tank is pH drop due to Nitrites. My reason for guessing this is that the tank is still cycling, but the nitrites still don't appear on my tests and I can see the presence of Nitrates.

I tested the tap water and it was reading at a normal pH so it couldn't have been from a water change.
Does anyone have any insight into this? What else could have caused this?
I added some sodium bicarbonate to boost the pH and the fish seemed to perk up a bit but another two passed away over night and the remaining fish don't look like they'll make it.

I should also note that my area has VERY soft water. I know that soft water can allow pH to change suddenly.
Does this mean I should be adding some sort of water hardening substance? Or would that make my water too alkaline considering the tank normally sits at 7.4? Is low pH the tank's new normal now that it has cycled?

Any advice would be appreciated. I'm devastated that my fish have had to suffer because of my ignorance. I'm feeling very discouraged and scared to get any more fish because I don't want to hurt them.

[Aiken Drum]

The nitrogen cycle uses up Carbonate Hardness (KH) in the process of turning ammonia into nitrate. 7ppm KH is needed to process out 1ppm ammonia. When KH is gone 2 things happen.

- The nitrogen cycle no longer functions.
- The buffering capacity of the water disappears and the other natural processes which acidify the water can no longer be absorbed and you see a pH crash.

In normal circumstances regular water changes, even with soft water, will replenish KH and stop this happening. If you are finding the KH in your tap water isnt enough to do this you may have to intervene with a source of KH.

Sodium Bicarbonate is a KH source. Other ways to add KH is a piece of cuttlefish bone in your filter or aquascape or a bag of crushed coral in your filter.