As per Bill's idea; I have removed fine sand/gravel with a 1/2 in. (internal diameter) hose too. Larger ones can be used but starting siphon is harder, they are less flexible, and can fill a 3-5 gal. pail very fast.

Get a piece long enough to reach from aquarium bottom to pail and after starting suction; direct hose to substrate piles. Initially moving all substrate to a corner with a blade less gravel cleaner or similar will facilitate removal.

Things to watch out for are that your pail does not overfill flooding floor and that the larger gravel pieces might clog tubbing. Good luck and have fun !