No test - just worthless stories.
Just because Capt Mike's tank is now algae free does not mean that the magnet is effective. The tank might have remained algae-free without the magnet, especially since he cleaned it fairly well.Anecdotal evidence is essentially worthless in evaluating products of this type, unless there are a LARGE number of reports. Similiarly, data provided by manufacturers is also highly suspect, since of course they have a product to sell and are unlikely to say anything negative about it.The only fair method of testing involves identical tanks, identical conditions, identical amounts of fuel, etc., with a magnet on one tank and no magnet on the other. Results can then be assessed via microscope.For my own anecdote, as valueless as any other, I have a very large magnet on the piping of my koi pond that someone gave me to try out. After 3 seasons of use, I get exactly the same amount of algae I did before I put the magnet on. I consider it worthless.I seem to recall that Practical Sailor performed a more scientific test of magnets as a way of inhibiting algae growth and also found them to be of no value.I'll just keep putting chemical algaecide in my fuel tank, thank you. I can buy a lot of that stuff for $ 100.00.MC