I'm making preparations to start my first Saltwater aquarium. Fish-only 40 gallon. I'm leaning toward an undergravel filter becuase its what i've always used in fresh water tanks and i know they are quiet. My question is; are protein skimmers and canister filters quiet? Since i live in a studio apartment i need to be able to sleep at night with the aquarium in the same room.

canister filters vary in noise level, not necessarily directly with price though. I have no personal experience yet, but have been doing research on them before I purchase. I have been told they are reasonably quiet, and if you can hide it in a cabinet it should not make much noise at all.

The protein skimmer I had for my salt water tank made about the same amount of noise as a HOB filter. When the tank was at the right water level the motor humming was louder then the water cascading. It did not make much noise at all. Fortunately you should not need to run the skimmer constantly.