Tcamos has the right idea but not the entire idea. I made a post on this where I explained the process.
Attach a new hose on your gravel siphon (once you purchased a large enough siphon) and run the hose out the window - 20ft or 50ft however long you need. This will get the water OUTSIDE and without a bucket or labor. Next, get the attachment at Home Depot or Lowes that attaches to your sink to allow for a garden hose; then put conditior in tank as it fills up. If it is saltwater, same process but skip the tap water.
For salt water get a 30 or larger gallon trash can and liners. Place a liner in the trashcan, now you can either buy a WATER TRANSFER PUMP or submersible pump. The submersible pumps are 1/3 of the price but require being in water where the transfer pump stays dry. The transfer pump requires a hose on each end, one in tank one in water and it transfers back and forth. If you go this route, you can use this to clean your tank as well as it will not suck up gravel (it has a stopper) and the other end of the hose out the window. If you use submersible you place it in your bucket at the bottom and fill your saltwater mix in it and then turn on pump to re-fill tank.
30-70$ depending on sizes and type of pump. Not too expensive.