When the aquarium is set up, you use a gravel cleaner to remove some of the water and clean the gunk out of the substrate. Then top the tank up with dechlorinated water. It doesn't matter what size tank it is, the procedure is the same for all aquariums.
For big tanks, use a garden hose or some clear hose from a hardware store and make the gravel cleaner out of a 1, 1.5 or 2 litre plastic drink bottle (like a Coca Cola bottle).
Use a pair of scissors and cut the bottom off the bottle and throw the bottom bit in the recycling.
Remove the cap and ring from the top of the bottle and throw those 2 bits in the recycling.
The remaining bottle is the gravel clean.
Stick a garden hose in the top of the bottle and run the hose out the door onto the lawn. Use the gravel cleaner to drain and gravel clean the tank. When you have removed 30-80% (preferably 50-75%) of the water, stop gravel cleaning and refill the aquarium with dechlorinated water.
To refill the tank have a couple of large plastic storage containers or buckets and fill them with tap water. Try to do this before you start draining the tank so the dechlorinator has time to neutralise the chlorine. Have the buckets a few feet away from the tank and add dechlorinator and water to them. Add an airstone to the water and let it bubble away while you gravel clean and drain the tank.
Get a small water pump like an AquaClear powerhead. Put a length of clear hose on the outlet of the water pump. Make a U shape out of pvc pipe (available from any hardware) and put the U on the outlet of the hose coming from the pump. The U will be upside down and hangs over the edge of the aquarium. Put the water pump in the bucket of dechlorinated water and turn it on. The pump will pump water into the tank and when the bucket is nearly empty or the tank is nearly full, turn the pump off.