Quaranteen tank

I have a 10 gallon tank that I set up to receive 12 Mollies and 3
Clown Loaches. Thursday morning I transferred 10 gallons from my 75
gallon tank. The Qtank has a sponge filter in a Whisper Jr power
filter. The sponge had been kept in the rear of a power filter in the
75 gallon tank until I moved the water and sponge. I added 1
tablespoon of salt and 1 teaspoon of Melafix.

The fish arrived Wednesday afternoon and appeared to be fine.
I covered the Qtank and didn't look at it until Friday morning. Again
the fish all looked fine. I fed them lightly and their was mild
interest. Three hours later I looked at the tank and it was cloudy.
The food had been eaten from the bottom, no sand, no plants, so I had
a good view.

This was seasoned water. The temperature was kept the same as in the
75 gallon. No new water had been added. How could the tank go cloudy
so soon and so quickly?

I had cleaned the tank with bleach early in the week, but rinsed it
with an outside hose. The tank had been standing empty until Thursday
morning.

I want to find a reliable Qtank procedure. Any observations,
suggestions?