I think I have diesel "bug"...
 

So the fuel tank in my SDL has always been spotless, even after sitting for 10 years with 1/4 tank of fuel, the inside of the tank sparkled. I use pump #2 only and I don't use any additives, occasionally I'll throw in a shot of 2-stroke oil, but that's about it.

Towards the end of December I wound up with a truly terrible tank of fuel. I filled up on the way home, and by the time I got to the house, the engine was already clattering like a sick tractor. The following morning, the cold start was really rough and the knocking was bad enough I thought the car had a rod knocking! Some "Power Service" additive helped a little bit, but it was still pretty rough as I used that tank up (takes me 3-4 weeks to use a tank with my driving habits).

Flash forward to now (about 6 weeks later) and I notice a few little black "chunks" in my prefilter. It's always been spotless since I put the car on the road in 2016, so I'm drawn to assume that rotten tank of fuel is my culprit.

I want to treat the tank, even if it isn't bug/algae/fungus/whatever just to be safe. I went to Vato-Zone and asked for a treatment and they looked at me like I was nuts. All they seem to have is injector cleaners and cetane boosts! I found treatments for ethanol and plenty of stuff for gas engines, but not a single biocide for diesels.

What's good, where do I get it, how often do I need to treat? I've never dealt with this before and I want it gone.

And yes, I went and complained to the fuel station I got the crap fuel from. Their response: "How do you know it was our fuel?" Won't be using them again.

I recently went through the same scenario of looking for diesel fuel stabilizer and biocide locally and gave up and ordered some from Amazon. Nobody had the stuff on the shelf around here. This is the biocide I got:

https://www.amazon.com/gp/product/B007D2C132/ref=oh_aui_detailpage_o06_s01?ie=UTF8&psc=1

If you drained out the tank of fuel, and treated the tank with biocide, starton, etc... fill, and change filters until the issue is gone... or pull the tank, and lines, and have them steam cleaned and kiln dried... reassemble and carry on.

Biobor is the only MB approved biocide. Makes sense to keep this in your trunk along with spare fuel filters and other emergency spare parts :

<a target="_blank" href="https://www.amazon.com/gp/product/B01LRDN9OC/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=B01LRDN9OC&linkCode=as2&tag=thewindycitywa tc&linkId=3a1c0bf0495d68fc4f316a24db761853">Biobor Jf Diesel Biocide, 4 Oz. BB04EZ01US by BIOBOR</a><img src="//ir-na.amazon-adsystem.com/e/ir?t=thewindycitywatc&l=am2&o=1&a=B01LRDN9OC" width="1" height="1" border="0" alt="" style="border:none !important; margin:0px !important;" />

I would think that Tractor Supply or other similar store that caters to farmers ought to carry biocide. Perhaps even a truck stop may have it. But yeah, sadly, I've found the vato zone to be next to useless for anything diesel related. Even Napa doesn't have it, not the one near me anyways.

Killem is another brand which seems to be very good, and seems to have worked for me. And keep the fuel tank nearly full to cut down on condensation.

The little black pieces that sort of look coffee grounds are supposedly asphalt which are metabolic products of the hydrocarbon utilizing microorganisms, which are fungi and bacteria.

Startron is very good as it dissolves all the bugs and they basically pass through filters and vanish out of the tailpipe , most other brands kill the bugs but they then block your filters.

I found out on gallon of regular gasoline to at least 20 gals. of diesel does no harm,and kills the bugs.

I have never heard thus... more likely the detergents in the brand you chose cleaned up some asphalt sediment...
Gas is still a hydrocarbon...

Boat supply will also get you what you need ,long sitting tanks is a very common water sports issue.

Oldsinner111 is correct, this is why most veg runners never have problems because they thin the oil down with gasoline.

Boat supply will also get you what you need ,long sitting tanks is a very common water sports issue.

I have also had outstanding results with Startron. There are diesel and gas versions. I haven't looked at the difference, always used the diesel version but note that the gas version is available locally in parts stores and small engine shops. Diesel version is available on line.

Dreaded Fungus
 

I've been through the mill with this in my Coupe, it was a beach car run on unfiltered canola oil with no fuel cap until it died......

Anyway, I began using Startron, I bought it @ West marine and as mentioned it in an enzyme that eats the fungus so it is very fine and passes through your system instead of clogging the filters .

I also used K100 : https://www.amazon.com/K-100-K100mg-Oz-Bottle-8oz/dp/B008Y2CH84/ref=sr_1_2?s=automotive&ie=UTF8&qid=1519054157&sr=1-2&keywords=k100+fuel+treatment

Works very well .

Be aware you may have to take the fuel sender apart and clean the fungus out of it as there are tiny little sediment traps designed inside the sender .

Get cracking on this before it's a huge problem .

I once got a tank of dino - diesel from Chevron that looked like COFFEE ~ as mentioned they didn't care .