If the sending unit is accessible on this like a W123 i.e., under the first aid box consider pulling it and try to look down into the tank before doing anything.

As far as an effective clean up product, sorry I don't know of any if the strainer is clogged. If the strainer is clogged there is likely more in the tank. I would use BioBor to kill anything in the tank before working on it much.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer