I think you are missing the back story. If the tank water pH is above 5.0 ( which it is), the nitrifying microbes that were there are still there. The fact that the ammonia is reading 0 shows that. That the nitrates are also 0 can be explained by the uptake from the plants since they are probably absorbing it faster than the microbes are producing it. That leaves the nitrites which may actually be a false reading. Without testing this theory first, doing any changes is premature. The OP's fear that they lost the biological filter ( cycle) is incorrect because the pH is not low enough to kill them off. They are slowed/ suppressed due to the lower pH but not gone. If they were gone, there would be an ammonia reading as well. Yes, raising the KH will help prevent these pH falls in the future but won't do anything for the current situation. Once the OP knows the actual KH level in the tank and source water, then a better water change schedule can be suggested.