You can personally only move up to a certain size. I wouldn't move anything bigger than 6x2x2 - you need to keep in mind that if YOU or your mates break it, it comes out of your pocket. If a company breaks it, they pay for it. Helpful equipment:
- trolleys with large, soft wheels (will make the job MUCH easier)
- glass suction cups (the pros usually have those when they move large tanks)
- temporary stands like wide, long, flat blocks of wood if you need to set the tank down on uneven areas
Biggest I have personally moved was 6x2x2 with just one mate - never again. ( I can't lift heavy anyway these days, bad back).
Generally for a large tank, 4 guys is the go. Plan your move - make notice of potential set down areas, potential hazards along the way like steps, potholes etc where people can trip or slip. Make sure the target area is clear and all angles are ok for the tank size. You don't want to go half way and then there is a corner you can't get around. And of course empty the tank as much as you can, ideally you just want the glass.
Most important: tell your crew your plan! Tell them which way to go, what to do if they need to set it down, and to speak up if they need to readjust their grip.
The largest tank I have seen being moved is my 8x3x2.5 - it has been moved twice, both times by pros who have done this before. The last move they had suction cups, scissor lift and 6 strong guys - it was still a huge effort to lift it up on the stand which is higher than normal. They had 4 guys lift one end up with suction cups, then change side and had all 6 lift the other end, then slide into place. A few selfies with the tank were taken afterwards to show their boss