How big a tank, and is it freshwater or marine?
Can you fit it in your car so it doesn't need to wait for the van? If it goes in the van, ensure movers are content with and insured for it and whether water will be allowed in the van and in what containers, or if you need your own in a car.
Make sure there is electricity on at the new house!
For my 150 litre tank I had plants and fish and filters in one bucket, ornaments in a bag, gravel in two buckets (heavy!!), and a dustbin with about 40 litres of water which was the most the movers would take, and about another 10 in bottles. Difficulty getting fish out near the end of the move and then all the gravel out, but the movers took it, then we had to drive immediately with them to new house leaving cleaner behind.
In new house, got cabinet and tank in place, plugged in, put gravel in, then the water I had, then added fish, then over a day added more warm water, with filters and pump added once enough water in, no light until next day. Took a few days for silt to settle but fish were fine. If I did it again I'd get a separate man & van.