Like others have said... Large diameter pipe and the tank full of water. Hose onle has to be bigger than the biggest piece of gravel it will have pass through it. I discovered the siphon method after moving two 6' long tanks that where full of sand/gravel. Since you say it's gravel you need to remove I would also recommend a bucket/trash can/rubbermaid stock tank and a cheap food strainer from the dollar store and a pump to push water back to the tank or to the drain if the need arises. An extra piece you may consider is a filter sock on the return to the tank so you can see better and not have to turn around and give the tank a good rinse after siphoning out all the gravel. Using the siphon method with a strainer also gives the gravel a pretty decent cleaning whilst removing it form the tank. I found a second set of hands to be most helpful and a game plan of what goes where and how. Hint, hint! Let the helper operate the siphon while you strain and bucket all the gravel. I have found they are more willing to help next time around. Good luck!