I have a tip to share. I have a weak left arm do to a stroke. So doing partial water changes on a 75 gallon is using a bucket is not an option for me. I thought I would give this a try and it works great. I bought a utility pump at home depot, its a Simer model 2100 1/4 hp 1320 gallons per hour portable/ submersible pump that typically is used for construction. My wife was stationed in the bathroom with the garden hose in the tub. I held the pump above about 4 inches above the bottom because I wasn't sure if the pump screen would keep the gravel out. She plugged the pump in. In about 10 minutes we drained 50 percent. Then I flipped it around. I filled a 5 gallon bucket up before submeriing the pump and kept the water running at a rate high enough so the bucket would not empty. My wife held the other end of the house in the tank inside a small 1 gallon bucket so the water would strip up the gravel. Plugged it in and filled it back up in about 10 minutes. The whole process from start to finish including clean up and packup only took about 30 minutes. I hope this tip will help someone else. Just be very careful when taking the pump in and out of tank !