My personal opinion is to try not to mess with water chemistry too far if you can help it. Directly tinkering with pH can lead to big disasters quickly.
As unclescott said, find out what your water supply pH is. I'd wager that it is on the alkaline side of things, probably in the 7.5 - 8.5 range. Also, measure your tap water before it goes in the tank.
If your incoming tap water is alkaline and your tank has ended up acidic, then it must be buildup inside the closed tank system that is causing it.
If your water is weakly buffered (KH) then just a little bit of organic buildup will cause the pH to drop rapidly. Higher KH levels give you more room to neglect things like water changes before the dissolved organic compounds etc cause the pH to drift too low, but it will still happen.
The byproducts of the nitrogen cycle tend to turn water acidic too. There are a lot of forces that are pushing pH downwards in an aquarium.
Anyway, if the incoming tap water is higher pH, it may be as simple as needing to increase your water change frequency. This can solve lots of problems. Note I said frequency, not percentage. If you're changing once a month, try every weekend (or two) and see how it goes.
IMHO try to resist the temptation to put things like 'pH up' chemicals in there.
BTW: check your water hardness too. Again, as unclescott mentioned, tetras like soft acidic water and mollies like hard alkaline water. Chemistry that one will thrive in will not suit the other. If you have both in the same tank, you're going to have to compromise on this. The reason I mention hardness (GH) is that I've found that my soft tap water (san francisco east bay area) causes all sorts of problems for my livebearers. I've had to add salts (either aquarium salt or various other salts)
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