Not all tetras need soft acidic water. And it is small tetra like fish that should be considered for 12 gal tank.
I would research,(google) the fish that interested me and see what recommended water parameters are for that particular fish. I have a ph of 7.4 out of the tap and moderately hard water. In a 58 gal tall tank I have kept or am keeping ,Lemon tetras,red serpae tetras,bleeding heart tetras,blood fin tetras,gold pristella tetras,threadfin rainbows,as well as yoyo loaches.
Is true that for breeding purposes ,many tetras need very soft water but for many, if breeding is of no interest,they can thrive in neutral to moderately hard water .
Many times the listed parameters in books for tropicals were printed when much of the stock available were wild caught specimens.This is not to say that the listed parameters should not be considered,just that much of what is available today, is farm bred and as such they have more and more become acclimated to slight variables with respect to their water chemistry needs.
Tank raised discus are an example. While they will adapt to slightly alkaline water, they may or may not breed in same. Wild caught discus would do poorly in this slightly alkaline water and perhaps ,or very likely, become quite sick. They are still wild caught and must be kept in similar conditions as that found where they are collected to do well in the long term/
With a Ph of 7.0 it is doubtful that water is very alkaline and there are many fish that would do well. The KH I would expect to be low but can't know without test results .