I am not stalking you if you noticed I replyed to all your posts in other forums, I just hit your search icon to see where all you had posted to kinda see where you were at with your big tank.
OK 1st, How long has the 210 been up and running? Does it have sand/substraite? You need something for biological filteration before you remove all the rock.
Yes you should take out all or most of the rock if you have sand and put it in a bucket/rubbermaid with a power head with the tanks water so the hypo will not kill off all the critters hiding in it.
You can put fake plants decorations or just PVC pipes in there for the fish to have somewhere to hide in away from each other. Especially if you have any bullies.
Next take out quite a bit of the water, I would start with 50 gallons on your size system as it takes a TON of fresh water to drop the salinity to 1.009. This HAS to be measured with a refractometer so if you don't have one, get one ASAP. Add FW in and keep a check on PH because PH will plumet when you drop the salinity. And buffer accordingly.
You can drop the tank right into hypo if you like as long as you keep up with the PH drop. Or you can do it over a few days so maybe the bio filter can try to keep up with the die-off of the things you can't see hiding in the pumps/cracks/sand and all.
Keep the SG at 1.009 for a minimum of 4 weeks 6 is better and raise the SG up slowley over a week by replacing top off water with salt water is a way I do it. Takes longer but it will get you there eventually, especially if you mix the top off strong.
The life cycle of ick will be halted with this methode but you must keep the SG at 1.009 to be effective. And you should really plan a QT (cycled) for all new fish you plan on getting as to not introduce another disease to the tank. I am dealing with velvet because I got lazy on my QT procedure and could loose hundreds of dollars worth of fish over it. Just lost 2 so far

. It pays to QT!
After its all said and done put the live rock back in. It will also be ICK free because it has had no fish for it to host on for the same 4-6 weeks.
Also if you have macroalge in the fuge/sump it will have to come out. LMK if you have any more ?'s...