Not to get too off topic; but ThRoewer, let's get hypothetical for a second.
Let's take the most commonly believed "difficult" (because anecdotal experience is all we got as of now) Indo-Pacific Regal. Let's also assume it's a good size for a Regal, 2-4 inches.
So based on that, if this Regal was added to a 4x2x2 120 gallon tank with dry rock and dry sand to start out with, how long do you think one should wait before it's safe to add one?
I would ideally want to have some algae in the tank for a new regal if the tank was set up with dry rocks.
Though if the fish is eating he should even be OK in a sterile tank for the standard quarantine period of 2 months.
My 2 smallest regals were (Bali)/are (Maldives) in QTs and did OK there.
I feel that one of the issues of the past was that too large specimen were collected and then not treated properly (= not fed well from day one). Feeding as soon as possible is key as they may stop being hungry after a while of fasting. Larger specimen may also have a harder time to adjust to captivity than juveniles or subadults.
I personally can't confirm that there are any regional differences in accepting food.
My Philippines, Bali, Sumatra and Maldives regals were all eating from day one and all I had to do was training them to dig in and stuff themselves during feedings instead of just nibbling a bit and expecting to get fed small bites all day long.
In that regard it helps to have some good eaters with them in the tank so that they feel some pressure to eat before everything is gone.
So to your questions: if the fish eats well it should be OK even in a fairly sterile tank. Though I would always recommend a more mature tank with a lot of micro life, some algae and ideally also some of the sponges they eat in the wild - ideally the tank should have been set up with good quality live rock. If wild collected rocks are not an option I would go with Tampa Bay's man-made and ocean cured rocks - from what I've seen and read they may be the very best you can get in the US.
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