I have taken some pretty nasty tanks with the fish. I drain 2/3 of the water, pull the fish and put in a bucket, hook up a siphon hose going out side and a python type hose to fill.
after scrubbing the sides I start filling and siphoning/vacuuming at the same time, keeping the gravel stirred up all the time, keep that up till the water runs clean when you turn the gravel over, takes about an hour or so on a 75g. filters off, heaters off of coarse. fill at water temp, prime the water, fire it back up and put the fish back in. What kind of filters does it have? thery will need to be rinced in either old tank water or water that's been treated with prime. not in tap water unless you are on a well.