I wouldn’t.
More than likely you will end up causing the Ph to drop and then bounce back.
Hard water has a buffering capacity the minerals in the water.
These carbonates will just bring the Ph back up.
You would be better to focus on softening the water by removing the carbonate minerals either through RO unit or adding peat.
RO would be my recommendation.
Also vinegar is basically a carbon source that could be added food for bacteria, which could cause a bacterial bloom in return leading to depleted O2 levels if you’re not careful.
Agree with this by BIG-G, before trying to adjust pH by adding vinegar (although it can be done), you may want to know the alkalinity (buffering capacity) of your water, and just how far to go before randomly adding acid.
Because it would need to be added with every water change to maintain consistency from day one, and may require more frequent changes to maintain that equilibrium, not just here and there at random.
One of the ways we determine alkalinity, is by dripping a weak acid (like vinegar) into a sample of water (titration) and watching for a color change, and extrapolating the value.
When the acid overwhelms the calcium in the water sample, a dye in the reagent changes color (green to yellow,or blue to red depending the indicator species used (Methyl-red, or bromocresol green).
In your tank, the normal buffering capacity of the water neutralizes acid, like uric acid from fish waste, keeping your tank water stable.
If the waste, in combination with vinegar, becomes too much, your tank could crash very quickly.
If your water is hard, with a high buffering capacity, there are plenty of tetras that have evolved to live in that kind of water, all tetras do not need soft, low pH water.
It may be that (as an example)wild caught cardinals or other black water species are not suited for your tanks, and making different tetra choices would be in more realistic. By adding tannins you might by pass acid alone, (a much safer method), but your tank water might end up the color of redish,or yellowish brown tea, like my tank seasonally does.
Any of the Mexican or Central American tetras, are generally hard water lovers, as are many of those tetras from west and north of the Andes, or far south of Amazonia (like Buenos Aires tetras), many of these have evolved to live in neutral to hard water, and will adjust easily. to varying conditions.
Another factor may be that Florida bred tetras, after being bred in ponds for many generation, since the early 50s, have evolved to tolerate higher ranges, and developed immunity to the bacteria that sometimes chronically effect wild ofish, or those species more recently added to the hobby
I currently have Panamanian Roeboides tetras in my tank, with a pH sometimes as high as 8+, and there are many other tetras that thrive here, in Panama and that water type. The pH drops seasonally in certain areas where tannins overwhelm.
One other thought, many small tetras are generally not very long lived fish, so the higher pH loving bacteria that often take a few years to chronically infect soft water fish in hard water, may be irrelevant (for cichlids that may live 10 years, a more chronic problem).
I don't know which tetras you have, but in nature small species like cardinals, or neons are often considered annual fish for various reasons, so living only a year is the norm, those in our tanks that make it longer, even only 2-3 years, are the exceptions.
