I've had success using freshly made salt water, which is my preferred method.
My concern with using DT water (especially because I already have anemones in the DT) is that there may be free floating organisms and/or biochemicals that may affect an anemone in QT. There's been talk in the past about allelopathy between anemones, and while these days I tend to think it's a non-issue and was primarily the blame for issues occurring when multiple anemones were in the same tank, I think there may be some merit in avoiding any such biochemicals when QTing a nem, especially one that is compromised in some way. I don't use carbon in my DT, so none of these chemicals are filtered out. If carbon is used, it may not be a concern at all.
Furthermore, we still don't know what causes in the infections in anemones. Could it be a free floating organism that attacks a weakened anemone? Could it be a combination of dead or dying zoox and an organism that is either attracted to, or feeds upon, the dead zoox and then attacks the anemone?
The reason I use freshly made salt water is to avoid the potential issues previously stated. My technique is to use a 32 gallon Brute garbage can to make batch of salt water. I use a heater to keep the temp at 80 degrees to my the QT tank and a strong pump to keep the water aerated. I simply take water from the garbage can each day. After day 4 I make a new batch.
The caveat to this is if you have a large system like OrionN. At 500 gallons, it's easier to just replace water to that system than to make water specifically for QTing a nem. And with so much water in the system, and I assume he's still using carbon, then we can go with the old saying...
"The solution to pollution is dilution"
