if it's a big tank you can make a gravel cleaner out of a 1, 1.5 or 2 litre plastic drink bottle. You cut the bottom off the bottle and stick a garden hose in the top and run the hose out the door onto the lawn. Then use the bottle as the gravel cleaner and clean away.
And of it's a real big tank, use the 2 litre plastic bottle with the bottom cut off, and attach some clear plastic hose so it fits on the outside of the top of the bottle. With this set up I could drain a 400 litre tank in a couple of minutes and gravel clean it at the same time.