They are loud the first two days, after they are broken in they are very quite if properly aligned. If not properly aligned, they are very loud. It takes very slight movements to get proper alignment and it takes practice. Make sure you get the cover, if you have a sick fish this thing can and will pull it into the blades.

Mounted on the sides walls this can and will make a wave that will go over the tank, be careful.

You can change the flow type to whatever you like: big wave, little wave, constant speed, variable speed.