OK here is what I would do Mark.
You will have to take all the decor out and leave it in the sink or garden whilst you do the tank and probably will rescape it anyway I'm sure.
You can actually just bag the fish and put them in a heated bucket and the heated spare tank but you could also put some of the fish in the spare tank, i.e. leave the main largest ones in the tank.
First you do need to syphon off the water, mostly because your gravel will need a really good vac, even if you do it regularly, once you start taking it out the tank you would be amazed at what bits have stayed under the gravel. You need to be slow and methodical and patient. Just remember to turn your heater off or it will melt and explode without water on it. I think by the time you've done the gravel enough to remove without making the water full of crud you will have probably taken out about a third of your water but this is wise anyway, a 2 foot tall tank is a nightmare to add/remove anything from let alone ALL the substrate.
You know those little pans you get with a brush and pan? Punch some small holes so water drains through but not gravel. This is a brilliant way to get gravel out of the tank without disturbing anything too much.
For your sand I can't stress enough wash wash wash wash wash wash it. Use a washing up bowl or, if you have a hose outside and are strong enough to lift buckets of sand, do it. This is going to be a long boring job, but very worth while.
The best tip I read on here was to spoon the sand into a tube but I think your tank's footprint is too large for this method, so I would scoop the sand into a 2litre plastic bottle with the neck cut off and then gently lower it into the bottom of the tank and slowly pour it out.
One other thing, if you intake tube is very near the bottom of the tank cut it slightly and raise the intake, otherwise it will suck sand into your filter and bugger it up.
I really hope this helps and hope it's not a nightmare for you.
Oh and also the gravel is heavy, don't put it in bin liners or they will split. The gravel will look good on the garden to keep moisture in the soil - in plant pots or on the pathway with pea shingle if you have any. Or offer it on Freecycle to someone to use it for this purpose but don't just tip it in the bin, cos it will be so heavy. I'm sure you knew that, but well, I should have known that too and didn't lol.
i rekon what i want to do is gonna be a good half a days work as my tank sits almost 5 foot tall on the cabinet my main concern is i want to do it in the least stressfull way for my fish
