If you have really soft water, then PH is likely to crash easily since there is next to no buffer. When the water is hard with lots of dissolved minerals, it is more stable PH wise.
The only way to find out is to get a test kit for water hardness.
I have fairly soft tap water in my area. Near the end of my fishless cycling, my water PH also crashed on daily basis while I was doing daily partial water change during the final days.
Fish can handle slow change in water PH even if the change is large. Since you have no trace of ammonia nor nitrite for two days straight, the fishless cycle is completed. Go ahead do a large partial water change before move your fish in. The nitrate needs to be lower. While nitrate is relatively harmless to the fish, high concentration is said to weaken their immune system in the long run. Aim for 20ppm or 10ppm, the lower the better.
On a daily basis, it is unlikely the ammonia produced by your fish will match the amount of pure ammonia you add to your aquarium during the fishless cycling . The PH won't crash that easily once you stop adding ammonia and put the fish in. Weekly partial water change is all you need to lower the nitrate and restore the water buffer.
