Thanks compress. I'm picking up the Startron tomorrow. Could only find the gas engine version today.
Ill refrain from using MMO. Sounds like it has a mixed reputation here.

I pick this up at a Marine shop near me. It does a good job of cleaning up the fuel tank without removing the tank strainer. It also helps with a jumpy fuel gauge. Notice it says good for gas or diesel. If you can leave the car for 24 hours for the enzymes to do their work and have some extra primary filters you should be golden.

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1977 Red 240D ( Squished by a 18 Wheeler )
1976 Black 240D ( Sold )
1991 White 420SEL ( Sold )
1987 Black 560SEL ( Sold )
1983 White 300SD ( Sold )
1984 Blue 300SD ( Sold )
2014 Red Chevy Cruze Diesel ( New Hotness )

I doubt 20% gas would hurt. I thought people in the frigid north mix 10% gas into diesel to aid starts, and once the engine is running it gets about as hot as in the summer. Also, many older diesel farm tractors would be started on gas. If there is a correlation with your bad running now, it is likely the gas dislodged black slime in the tank, which is now clogging the screen at the outlet. Biocide may digest that.

Years ago, my wife called saying our 1985 300D "sounded funny". I drove there, and could smell gas in the tank. I asked if she remembered to always use the "green pump". She forgot. Drove it home 3 miles, then used the gas in my gas cars & refilled diesel. The engine failed about a year later, but unlikely related. Something round like a ball-bearing was bouncing around in cyl 1, leaving marks, and no idea where it came from.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

That gas version is fine to use if you have to. The diesel blend is more concentrated. If you used more of the gas bottle in the tank it would have the same affect as the diesel.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

Woory about the algae, don't worry about the guzzoline. We're talking about a mechanical tractor engine here.

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$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment

Bacteria, not Algae.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I wouldn't worry about the gas, only harm it will have done is clear out some gunk.

I also wouldn't run any complicated cocktail of chemical additives as a shotgun approach with fingers crossed. You've already purged the gasoline from the system, so it won't be causing further damage(if it did any to begin with), and nothing you can add to the tank will fix any damage done. Run a specific product for a specific use and leave it at that. Startron for your algae/fungus/whatever, and a lubricity additive like two stroke oil or biodiesel if you don't already.

Like Mach4 I've burned some gasoline intentionally, getting rid of 2 stroke gas that I didn't want to dump in my cars, a little at a time. No harm done.

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617 swapped Toyota Pickup, 22-24 MPG, 50k miles on swap

Running 2 stroke gas in a gas car is less risky / likely to cause a problem than running it in a diesel. ( and small amounts of gas in a diesel isn't an issue either ) A gallon of typical 50 to 1 ( 2.56 oz oil per gallon ) mix is going to be pretty thin mixed with 15 gallons of gas. 800 to 1 or so.

It appears you guys are correct. At this point, the fuel tank sludge is the clear culprit. I'm having to change the prefilter every single day. When a new filter goes in, it runs great- until the filter turns black after 40 miles. So I'm going to have the tank screen pulled and cleaned, but leave the tank alone. Then add the Startron and maybe sea-foam too. I just need to slow the amount of sludge coming from the tank.

If it's that bad drain the fuel and drop the tank and pressure wash the tank out. Let it dry and change the filters and blow compressed air thru the lines.

Pulling the tank screen requires draining the tank (and I hope you don't plan on putting that fuel back in). Once empty, it's a just one more step to pull the tank and have it steamed out. Bio activity is self limiting...it dies out when all the water in the fuel is consumed. So you're probably buying fuel from somewhere where rain or groundwater is constantly seeping into the storage tank.