Here's what I did to clean my W123 tank of rust and black sludge.
Drained and removed it, took it to the local car wash and used the high pressure wand to blast out what I could from the sending unit hole. Then poured in a couple of gallons of hydrochloric acid, sealed it up and rotated it in all directions. In about 15 minutes took a look, it was perfectly clean inside. Rinsed well with a garden hose and then poured in a gallon of drain opener (lye) to neutralize the acid ions. Rinse well again and poured in/swished a gallon of denatured alcohol to absorb leftover water. Then dried it in the hot sun. It did not flash rust.

You can see quite a bit of the tank through the fuel gauge sending unit hole with a wand flashlight stuck in there, just don't drop it!

Pre filter remains perfectly clear.

I replaced the male end on my garden hose a while back. I removed the male end an then plugged the open end with a 3/8" to 1/2" ratchet/socket adapter and hose clamp. After sealing the end of the hose I drilled a 1/4" hole in the hose about 2 inches from the end. It made for a good rinse.

The tank is black and gets hot to touch after half an hour in the SC Sun.

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84 300SD
85 380SE
83 528e
95 318ic