A friend of mine, owner of a marine supply store at the time, was having a lot of trouble with his fuel system, so put in Algae-X. That was some 20 yrs ago. Since doing so he had his tanks "polished" and has tried other chemical additives. He has now sold the boat. Despite his struggles to get clean fuel to the engine, he never succeeded. For various reasons, his annual hours on the engine never amounted to much.
By contrast, in the same time frame, I have consistently put 150 hr a year on mine. The last time I changed filters in the Racor 500 primaries, my log told me it had been 4 years. At the same time, I changed secondaries, and my log told me it had been 12 years. I didn't buy the Algae-X.
Also, in the same time period, US EPA rules for fuel have mandated Low Sulphur diesel, so the quality of filtering before the fuel gets to the fuel barge has improved significantly. Add to that, in Vancouver we have lost 4 of the 5 fuel barges that lined the shore of Stanley park, so the Chevron barge now supplies most of the commercial craft in the city. Seaspan alone pumps millions of litres of diesel through that one barge annually, so there is no danger of fuel stagnating.
To find any effect of Algae-X in this environment without rigorous scientific testing would be impossible. OTOH, the quality of fuel has improved so much since A-X was introduced that its need has disappeared completely.