Ph is important and needs to be stable.
Unfortunately, there are many other factors that are just as important and can be lethal to your fish.
Fish waste and uneaten food etc cause a chemical called ammonia build up. While this builds up over a week or so, a bacteria starts to form in the filter and gravel that eats the ammonia and converts it into nitrites. During the ammonia build up though before the bacteria starts to grow, levels can rise and kill your fish unless you change the water enough to keep its level safe.
Once there begins to be nitrites in the tank(also lethal) another bacteria forms over a few days or so to consume the nitrites and convert them into nitrates. Nitrates are easier to manage with normal water changes.
Ammonia>Nitrites>Nitrates.
You need a liquid test kit to monitor these levels. In a cycled tank ammonia and nitrites should be 0 and nitrates under 20. Untill it cycles you might need to change the water daily.
