Chlorine a concern?

Tanks! said:

Do well water softening systems (ours uses a product called Solar Salt) put chlorine into the tap water? Should I be treating new tap water with dechlorinator?

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dechlorinator, probably not, but then again you rarely suffer for using a water treatment product either.

solar salt is regular sodium chloride, at very low concentrations it is frequently argued that it can be beneficial for freshwater fish

Using salt in the freshwater aquarium - Algone
while i don't necessarily agree with the resulting conclusion of the page, it does very thoroughly discuss the topic from both sides.
i would argue table salt is a terrible thing to add to an aquarium, however aquarium salt (potassium chloride not sodium chloride) can have some positive benefits under specific conditions. though should be avoided in the planted tank. i once bought a water dechlorinator with "fish aid" that killed nearly 300$ worth of plants in a single dose. it wasn't until after i researched the product in detail that i discovered the "fish aid" was regular sodium chloride not potassium chloride as i had assumed.

a warning of note, water softeners usually make higher concentrations than are beneficial for freshwater fish, you would need to evaluate your own water to determine its potential for good or bad.
salt at any concentration is much more detrimental to planted aquariums as freshwater plant species are, broadly, more susceptible to osmotic pressure
and never ever use iodized salt (not really an issue in your case, but still... i'd hate for someone to do that to their fish)

it is possible to change the salt filter of most water softeners with a potassium filter (they're more expensive which is why most people don't use them). the effect on your water will be the same, but it will be more safe for aquariums.

Water softeners are like many things we use. Often talked about and often by people who have only heard stories rather than knowing. Softeners do add salt ions to replace calcium and magnesium ions. But the system then also does a rinse to remove most of the actual salt that is used. You may put a sack of salt in each month or so and that makes people believe they are drinking/eating a bag of salt. WRONG! The salt is made into brine which is used to do the ion exchange in that type of softener. Then it is rinsed down the drain.
Step one is to know what type of softener is used. There is not a separate type to use potassium but the same softener with different settings and you add potassium chloride rather than sodium chloride. If one is in difficult health conditions and the doctor recommends not using any salt, it is practical to switch to using potassium chloride but it will cost far more. For most people, it is not worth the expense as the amount of salt you take in is somewhat close to the amount you can cut out by not eating a slice of white bread! Kind of silly to pay high prices for potassium while still eating a sack of chips! Looking at the labels on food will show that almost all processed food has more salt than softened water. Good old chicken noodle soup is deadly when comparing the two.
For plants, there is often talk of not using softened water for the yard and many houses are plumbed so that the outside faucets are raw water. But the primary reason for that is the expense. It is silly to pay to use soft water on the yard. However, it is also not harmful to most plants. I have added irrigation on one side of my house and it was convenient to plumb into the soft water. So the flowers and grass on one side of the house gets soft water while the rest gets raw water. We do a lot of irrigation here in Central Texas and the salt does not harm the plants. People near the coasts get far more salt spray and they do have problems with growing things as it saturates the ground.
Normal softener setups do not add chlorine, however if you live in an area where iron in the water is a problem, chlorinators are sometimes added to the treatment to help remove the iron. So a bit of looking/ study is needed to know what the water has.
Bypassing the softener will work to get raw water but it does take some thought to do it. When the softener is bypassed, the water does go past rather than through the softener. But the point often missed is that the line between the softener and whatever faucet is used, will be full of soft water until that water is used and replaced by raw water. So it may take some time to get the raw water to that point. For hot water use it is really not practical as there are 30-50 gallons of soft water in the water heater in many houses.
I would recommend finding out what your water has and then decide. I might recommend looking for a way to use the raw water from a faucet before the softener.

Abbeysdad brings up a point that should seem obvious but is often missed or ignored. We all know that there are many many ways for ground water to be polluted so using a well as drinking water does need to have some caution added and it is more important for personal wells to be tested than city supplies. If you think of the underground as a big sponge with multiple layers and many many channels for the water to move from one layer to the next is is easy to see that something in the ground miles away can suddenly break through at some point and wind up in the well.
At one point the US went through a phase where public safety in the water supply was a big concern and lots of info was developed to find and list the major polluted spots. Those were named as "superfund" sites as money was put into a super fund for clearing those sites. But over time and with changes in political climate, those funds were diverted, leaving many of the superfund sites still in the ground and some removed from the list. That doesn't mean your water is safe, only that we don't talk about the problem!
So if one uses well water for daily drinking, I consider it very important to get a test done at least yearly to avoid the long term health problems that it can cause. Killing your fish may be the smallest part of the question!
This is a link to some of the info on superfund sites so that you can locate them in your state but keep in mind that pollution doesn't just come from your area but can come from hundreds of miles away in almost any direction.
https://www.epa.gov/superfund/search-superfund-sites-where-you-live