Both Prizm and Skilter 400 are lound
but with a little mod, they are perfect for small tank like 55 Gallons
I have MOD them both very happy with the price and performance
noise are made by air bubbles making direct hit to the champer
make mod so that there is same amout of bubbles enter the champer(to keep your performance at peek) but do not let them direct hit to the wall( less noise)