Use cable ties found in the electric department of any Hardware store or even Walmart. Might want to rinse them under the faucet first, but I use them for alot of things in and out of the water. The sponge is not a bad idea, it also acts like a wet/dry filter, kind of like a bio-wheel that doesn't spin. But the best way I found to silence the waterfall cascade type of noise is by keeping the water level of the tank to the bottom of the filters return. Can't even hear it. When you do, time to top off the water. A glass canopy with the trimable plastic on the back, trimmed tightly to and needed openings with an exacto knife and drill, really help keep the water from evaporating. It also holds in heat so the heater doesn't need to work as much.

I find myself topping off the water in my 10 gallon in the bedroom once a week, when it gets loud. I can't get to sleep to a loud GURGLE GURGLE.

And we moved the aquarium into the bedroom because the noise helped us sleep. Curious world isn't it?