I don't know enough about water softener to advice taking water out in the middle of the process!
FAIK, you add soda ash (Na2CO3) to increase your water pH. This is done so there is less corrosion in piping & for water softener to work properly. My city water co does the same (but using
[acronym="Sodium hydroxide"]NaOH[/acronym]). Now, I would have no problem with the the water treatment plant adjusting pH. They test frequently & results posted daily on the web - and the pH is kept to within 0.1. I would not have such faith in a home setup. Any change in the Na2CO3 injector or in the incoming water would result in different pH, and for fish, the worst thing is a fluctuating pH - the reason that home hobbyists are adviced to acclimatize their fish to the pH of their tap water rather than doctoring their water themselves.
At any rate, I don't see any reason to take water out after the soda ash injector. Adding Na2CO3 will *increase* the
KH (even though the
GH is unchanged). For the fish that means harder water than your original tap water. Since the water softener won't soften the water from the fish's standpoint, so it really makes little difference taking water before or after that.
My feeling is, if you have problem taking water before the water softener <note that if you have an outside tap - that is ususally before the softener>, just use the water and don't worry too much about it. Most fish will adapt ..... and if you are keen on some exotic fish that needs real soft water, then you'll need to buy a
RO unit. If you are keeping snails & worry about the
Ca level, just add some to the tank in the form of crush coral, shells, etc.