[ThRoewer]

I see no problem in using tank water if it is properly disinfected.
I have currently some fish in hyposalinity for Ich treatment. For water changes I take water from the main tank and boil it. Then I adjust salinity and temperature by adding RO water and ice - works perfectly fine. And the corals in my main tank need the fresh water more than the fish.

I would also do TTM under hyposaline conditions, firstly to reduce stress and strengthen the fish's immune system and secondly to reduce costs.

And hyposalinity is recommended for copper or antibiotic treatments too. Especially copper treatments should be done at reduced salinity since copper severely reduces the fish's immune defenses.