If you do a search on AC for "Python," you should find what you're looking for.
Essentially it hooks up to your faucet, you turn on the spigot and then use that water pressure to drain the water from your tank. Then, to fill your tank (or a reservoir if you prefer), you just close a valve on the faucet end and let the water flow (after you've matched temps, of course). No buckets.
Do the search, though, and you'll find more information, along with many glowing reviews. It's one of the best aquarium-related investments I've made.