OK Caper, this is how ALAN would approach it.
First, get enuff water to do a complete change and store aside, so it will age.
In your case you may have to use those chemical neutralises that you need for your water.
While this is aging, I would now locate some 1/2" (10mm) poly, cut it to size and have reasonably handy.
Now on the day of putting the poly under the tank, I would catch the fish and put them into a bucket with half their tank water and half the now aged water, that should be the same temperature as the aquarium water.
PUT A COVER ON THE BUCKET
Now using a vaccuum do a real good job of syphoning, and thereby cleaning, the base gravel.
That should get rid of two buckets of water.
Throw it out.
Now syphon the rest of your tank water out into specially designated fish buckets, (not used for anything else but fishy things) until you have three or so buckets of saved tank water.
Discard the rest. Or save that into other buckets where your gravel can be finally washed with it.
The tank is now empty and can be moved without fear.
Move it to a safe place and put you poly where the tank use to be,
replace tank after a thourough cleaning of the glass and replace the now spotless gravel, filters, etc.
Put into the tank the new water, about half, then add the old tank water that you saved,
being careful not to get the lees that will have settled out by now.
Catch your fish from theit impromtu prison and release them into the aquarium.
Now turn on the electrics.
Heater, filter, air-pump and lights.
Give the front of the tank a clean with a towel, and then clean up the mess of empty buckets, hoses, nets,
and anything else that you used for this major move.
Now go get a can, crack the scab and sit back and admire your handywork from the comfort of your eziboy chair with your feet up, knowing that after having visited the FNZAS.org.nz site, and having friendly folk advising you,
that you'll now set up your next tanks with poly under their bases.
Alan 104
ps.
if ya can't figure this out from what I've written, PM me for my address and I'll send it to you,
then you can send my return plane tickets and I'll come and do it for you.
Al