Whatever you buy, look at the db rating. That would be decibels. Something around 23db would be pretty good, as they'll do the job quietly. The issue is that they don't move as much air.
Adding more fans that are each 23db might work, but then again you would have 2x23 or 3x23db... and the combination of several may make it louder. I've run one or two computer fans on each of my tanks over the years and they were very quiet.
The IceCap fans are designed for the saltwater environment, and when on low you don't hear them at all. There have been plenty of nights when I was wondering why the tank was getting so cold (79F or less) and I watched the heaters click on. Usually the next day I'd realize I'd left them on and figured it out. During the day time, the heat sensor speeds the fan up to medium or even high. You can move the sensor to the perfect spot to get the low speed all the time or maybe behind the tank for when the heat reaches its peak for the day.
