Drain tank to almost empty into buckets, remove fish, replace water. Done in 20 minutes. It's good to have someone there to help with the process. Also have a bit of new water made up like 5-10 gal for displacement and do a water change then. You may have to take down the top set of rocks...it's also a good time to give the rocks a bit of a cleaning of detritus by swishing them in a bucket of water.{use some of the old water, then replace that dirty water with the new water.} Good luck.