There are plenty of people who use them, including Steve Dashew.
https://www.setsail.com/preventing-fuel-algae/
I don't use things I don't understand, and I don't understand how a magnet can affect fungi such as hormoconis resininae (diesel bugs).
Concentrate on keeping the water out of the fuel. The bugs need water to survive.
btw - Interesting boat, - Are you planning a singlehanded circumnavigation?