Can't change water due to quality issues, need to move fish to another tank

I think my recovering fish in the hospital tank needs to move back to the 125l qt asap. I have not been able to change the water over the weekend because my tap water is still suspicious after the maintenance work was done last friday. The hospital tank is only 40l and I'm afraid there will soon be ammonia/nitrites in the water as I'm now also feeding more than before because the discus has started eating better. Also the Camacell treatment needs to end (going on for the 8th day now instead of the recommended 5 max 7).

My tap water pH is currently 6,2 straight out of the tap with visibly lots of dissolved gases. It clears up after aging a few hours and the pH comes up to 6,6 but I still dont trust it. It's normally about 7 straight out of the tap and comes up to ~7,4 after aging. Very small swing (max 0,2) when adding into the tank directly from the tap.

Both hospital tank and the bigger qt tank have pH of 7,1 and temp at 31C, I think it would be safe to move the fish between them but the bigger qt has been empty of fish since thursday (4 days) and water hasn't been changed there either. Normally I would mix the water in the hospital tank, adding about 30% from the bigger tank and replacing with tap water in the bigger tank to make sure the water matches before moving the fish. But obviously now I can't do that either because I can't replace the water.

What would you do, just move the fish without any water change and hope for the best or keep it in the hospital tank longer still, no water changes and the remains of the Camacell treatment also in there longer than should be? Or should I just do the water change and overdose with dechlorinator (Prodac Aquasana)?

Can you use the water in the qt to do water changes in the hospital tank? You could even use that tank to age water and filter in tank through carbon or a resin to remove chemicals before a water change.

Technically I can but that water is even "older" than in the hospital tank since it has sat untouched 2 days longer. But it would be free of medications.

But yeah, what if I do one water change now to get rid of some of the meds in the hospital tank and replace it with tap water and start filtering through a carbon sponge? Then it would be a fairly small amount of new water that goes in initially and it should be filtered and safe by the time I have to do another water change.

// This is what I did for now. Changed about 15l using the old water from qt tank, replacing with new and adding carbon. I can do another very small water change tomorrow and if the fish looks good then increase the amount of new water that goes into the qt tank for aging and carbon filtering. Hopefully in a few days it'll be good enough to use straight from the tap.

Thanks again for the advice, Al, very much appreciated!