Poor redsea's head has got to be spinning by now
Using DT water to fill your QT is fine ,and even better..The temp ,PH and salinity is a perfect match ...Once you start hypo it doesn't matter anymore(already stated above) Hypo should be used over meds IME and IMO
(if being treated for ich and only ich) ,,much more relaxing for the fish and 0 risk of over dosing..QT can be left empty ,but not a must...i say put a couple rocks in for the fish and i do..
I have been doing the same QT and hypo process for 6 years
Ich does not die in 2 weeks and has been thought to live for up to 6 weeks.
Leave DT fallow for 8 weeks