I think there are a few days to think about this.
For one, 6 hours is a long time to stress the fish with your hand in there, rockwork moving around, and with elevated nitrates from stirring up the sand and rocks. There's also the chance of crushing/injuring one of them during the rearranging.
The other thing to consider is, even if you had a holding tank, you should still wait longer than just 6 hours before transferring the fish back in, some of the debris you've raised out of the water will cause a change in water parimeters, and you'll have to wait for these to go back down.
In an established tank, it doesn't take a lot to cause large swings in water perimeters and hurt your livestock.
If it's a simple rearranging, then sure leave them in, and just move slowly. Turn off your powerheads so if you do stir anything up from the sand/rocks it doesn't end up affecting your entire tank too quickly.
In my experience (with FW
, not SW
), I usually use a holding tank when I want to do a complete tear-down and re-set up again.
If this is the endeavour you wish to take, then I would do the following:
1 - Make new SW
and keep it in a tub, enough to do a good 40%-50% water change after your rearranging. Make sure your water is buffered, and to make this a few days in advance.
2 - Fill up another tub, large bucket, or holding tank with water from your main tank. Put a live rock in there for hiding, a heater, and an airstone in there. I wouldn't worry too much about a powerhead / water flow, especially in a bucket (cause a whirlpool effect)... remember, when they were transported from the ocean they were in bags for hours. Keep your holding tank in a dark room somewhere to keep them more relaxed, without much commotion going on around them.
3 - I would remove the rocks (keeping them moist), then clean the glass, then clean the sand bed. Also go through the filtration, cleaning out the tubes, media, etc.
4 - Put the rocks back into the aquarium. Turn the filtration back on.
5 - Remove as much water as you will be replacing with new SW
in the tub. (40-50%)
6 - Wait around 2-3 hours for the water to settle
7 - Introduce the new SW
into the aquarium
8 - Test water perimeters, ensure your pH, salinity, ammonia, nitrates, nitrites are similar to the original perimeters in the holding tank (or better).
9 - If there is a large deviance in the water perimeters, then wait hours (a day?) until the aquarium settles, it may be a good idea to re-acclimate the fish to the aquarium again if there is a big difference in water quality, instead of shocking them when you drop them back in again.