FWIW, the Litermeter while being a great dosing pump is not particularly quiet and I don't really consider it continuous duty in that it needs it's rollers cleaned somewhat regularly depending on how often it's running. I used a Litermeter 3 for a few years to manage my automatic water changes and while I love the Litermeter, the maintenance and calibrations every few months was my only gripe.
The Cole Palmer pumps are arguably one of the best continuous duty pumps and while I have no first hand experience with them, as mentioned above, they are fairly noisy according to those that run them.
For my ATO pump, I use an Aquamedic dosing pump. I went that route as it is slow and if I ever had an issue with a float switch, the reduction in salinity would be slow enough that my failsafes would alert me long before my salinity levels were even close to critical. While it's not really a continuous duty pump, it needs little to no maintenance and is virtually silent. They are also cheap enough that I keep a spare pump and hoses for easy service and maintenance. The only real maintenance they need is the hose being changed every several months and an occasional greasing of the rollers which can be done while the unit is in operation.
