You need to make up a long gravel cleaner to drain the tank water out the door and onto the lawn. You can do this with a 1, 1.5 or 2 litre plastic drink bottle and a length of garden hose.
Cut the bottom off a plastic drink bottle and throw the bottom bit away.
Remove the cap and plastic ring from the top of the bottle and throw those 2 bits away.
Put a garden hose in the top of the bottle and run the hose out the door onto the lawn.
Use the bottle to gravel clean the tank and drain the water out.
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Get a large plastic container (storage container, wine barrel, plastic wheelie bin) and put it near the tank. Fill the container with tap water, add dechlorinater and aerate the solution.
You can use a garden hose to fill the container but run water through the hose for a couple of minutes before filling the container. This will remove any softening agent in the hose.
After you have drained the tank, use a small water pump and some plastic hose to refill the tank. You can use an AquaClear 402 or 802 power head, some clear plastic hose and some pvc pipe for this purpose.
Make U shape out of pvc pipe.
Put some clear hose on the outlet of the power head and fit the other end to the pvc U. The U hangs over the edge of the tank and will hold the hose in place so water goes into the tank.
The power head goes in the container of dechlorinated water. You turn the power head on and let it pump water up into the tank.
When the tank is full, you turn the power head off and pack the stuff up.