WOW! I think I can actually help you out here. Ironically, in april of this year, I did the same thing, only we moved from one end of our apartment building to another so that we would not have anyone living underneath us because we have some very loud kids!
Here is what we did:
our tanks are both 55 gallons, one with a bunch of platies, and one with bala sharks, pleco, tinfoil barbs, rainbow sharks, couple of peacock eels, and a snail. (Yes, it was overstocked and has now been taken care of). Anyways, we faced a couple of problems; it was pretty cold out and we had to take all of our stuff out of the building in order to get to the new apartment. Also, we had only two days to do it all.
We went to a local shopping store and bought 4 ten gallon jugs that are used by people who travel with those big trailer homes. This meant that we could take 40 gallons of the 55 gallons of water in the tank with us. We boiled water in a big pot and used a siphon to put it in the jugs to clean them out really well (the sihpon was shot at the end, but it was worth not having to recycle a tank). When the moving day came, we first made sure we had all the furniture over in the new house so that we were sure everything would work out once we had the tanks in place. Obviously we got the electrical outlets all figured out and what not so we could just plug things in and go.
First, we took out about half of the water from the tank so we could catch the fish easier and safer. (We also bought the biggest net we could find to help us catch them quicker). Then, filled up a 10 gallon pail with more water from the tank and placed an airstone in it to keep the water breathing. We quickly put the fish into the bucket and covered it so none could jump out (this was an after thought and trust me, they will try to jump out). Then, we filled up the rest of the jugs that just held water and let the rest of the water siphon out side or into the bath tub. (We used a couple of pythons to help speed this up). Then we moved everything over to the new house as quickly as we could.
We set up the tank without the gravel or decorations and put the water we saved in the tank and got the filters and heaters running. We put the fish (still in the bucket) in our bathroom near the small heater vent to keep the water warm. This gave us enough time to get the gravel cleaned up, back in the tank, plants set up with the decorations and the water in the tank back to between 72 and 80. We put the fish back in with a small amount of aquarium salt to help the fish deal with their stress levels and I swear that we didnt lose a fish or have any problems after the move. (I was really worried about ICH since we have had problems with it in the past).
We repeated the process with the other tank and it took us about 4 hours to get both up and running again. We were also able to clean up the gravel really well and rearrange the tanks to make them perfect. I am sure that us taking the water with us over to the new house was what saved us. The only other advice we had was to basically 'clone' the tank again without taking the water and rely on the bacteria in the gravel and filter media. This probrably would have worked also, but the extra money we spent on the water jugs was worth the reduced risk and we still use them for water changes now since we cannot just run a siphon hose out the front door anymore like we could at the old house.
I am really sure that the key to moving a tank is really doing whatever you can to reduce the stress on the fish. We try to feed them well and add some of that aquarium salt to help with their slime coats to protect them from ICH and other disease. Also, we are pretty new to the hobby and didnt know exactly how we should go about this type of ordeal. Our move was also pretty sudden since we had to make a snap decision to move or stay in a home that our kids had to basically sit on a couch as much as we could make them in order to keep the people below us happier.
I am sure that if you preplan and keep in mind many of the 'rules' of keeping a fish healthy and stress free, you will have as much success as we did! Good luck
One more thing I forgot to mention, when we moved our tank, we took most of the substrate out before we moved the tank. Just like Tolak mentions, the tank glass is designed to bear weight that is immobile or does not move rapidly. Also, I have heard about people who use a board to support the bottom of a tank in order to move it. My thinking is though that the board is supporting the bottom of the tank itself and not exactly supporting the glass. I would be worried that the shifting weight and possible twisting as you went in and out of doors and across a parking lot could weaken the glass or plastic and break it.
