What happened is likely "normal" but you do have a real problem here.
First, can you give us the tap water hardness? Check with your water supply people, some have websites with water data posted. We need to know the hardness numbers, which can include GH (general hardness), KH (carbonate hardness), alkalinity, or mineral numbers, whatever is available. This will allow us to determine the pH buffering.
But aside from the above, you have fish requiring hard water or they will not be healthy. Livebearers. If the pH went down to 6 due to a week or two of neglect, it suggests the hardness is low and the increased organics acidified the water easily and significantly. You should increase the water hardness [even a pH of 7 is low and it is liable to drop easily depending upon the hardness], and we can discuss the best methods once we have the hardness numbers. It is much easier to safely and naturally add hardness than lower it.