It is not a good idea to transport a tank with water, or any kind of other weight in it, like substrate, decor, etc...
Tanks are designed to hold water/weight and be stable in a static position. If you start moving the tank about with weight in it, the water/substrate will not exert a force equally in all directions.
When water is inside something (a tank) it exerts a force perpendicular to the surrounding container. When you move that, the "water weight" will be shifting all over the tank, because the tank is not in a static position. This will cause the tank to break, or worse, catastrophic failure.
Besides, the water does not hold the beneficial bacteria. The filter does. So make sure you keep the filter media wet in aquarium water. Get a bag and fill it wilth 1/3 water and 2/3 air to transport the media and fish.
Yes, just leave around 15-20% of the water, unplug and preferably take out all the equipment, and *perhaps* some of the ornaments/toys, and you should be JUST FINE.
Just think of it this way: When my family was travelling, and I wanted to give my female Betta fish to someone reliable, I left around 15-20% of the water in, unplugged the heater and took it out, left the fish in there and then put her on the floor of the back seat and we just drove her 40 minutes out to the town where our friends live. XD
For future reference, remove the fish and place them and some of the tank water into a cooler to transport them along with your filter media. Remove everything from the tank! FHM knows what he's talking about.
When I moved country with my fish in mid winter, I bagged all the fish into fish bags and those into poly boxes, then got as much water as I could out of the tank into more bags (to pad out the poly boxes) and buckets with secure lids. Drained the remaining water and packed all decor including substrate into buckets (which I use for water changes). And drove for 24 hours. I also used heatpacks to keep the fish warm, but you should be fine if you wrap the bags in towels for the journey as it is not that far.
If you leave anything in the tank, there is higher chance that it may break then if you take everything out.
When we moved it wasn't far so I put all the fish in a big rice bucket with some of the tank water. Moved the tank and fish the day before we actually moved. Was only a few streets away though! Took the 3ft tank on the back seat of the car and set it up and put most of the water from the bucket, just leaving a few inches for them. Then filled upw ith de-chlorinated tap-water and put them back in. All done in 2 hours. They were fine and no losses. Wasn't risking the tank in the removal van.
Hey, I just moved into my house about a month and a half ago. What I did, was drained out 10-12 gallons of water, so there was less than half left... Then I pulled out the scenery, and all the plants and heaters, etc...
Then I went fishing!!!! Got the net out and scooped em out and into a 5 gallon home depot bucket with fresh water, with the plants and driftwood in. Then I put a lid on, and drove to my new house. Got the tank all cleaned, freshened up, and added the fish to their nice clean tank!
To be on the safe side, I recommend properly packing them up and not leaving water in the tanks.
What I did when I moved my fish:
Got a cardboard box and lined it with sheets of polystyrene. Then I individually bagged the fish with tank water (so I didn't have to acclimite them to fresh water) and made sure it was half water and half air, so the bag was 'plump' when I tied it off. Wrapped each bag in newspaper to keep warm and keep the fish calm, and added them to the box, packing around the bags with newspaper and polystyrene chips. Once all boxed up, the fish were fine for about 5 hours, so doing this would give you plenty of time to unpack. It also kept the water quite warm.
The tanks were fully drained and all the plants were also bagged up with some water, as was the filter media.
The tanks were transported with only the gravel in.