Black "rubber like gunk" in diesel tank

[Peter Morgan]

When cleaned out diesel tanks, transfered through filter from one tank into other, then back again, last cleaned approx 2 years ago. Diesel ok, some bits in tanks but otherwise clean. No sign of bug. Only thing I found which puzzled me was a small black ball, golf ball size, of a rubbery like substance, one ball in each tank, . Diesel is treated with bug "killer"
Any ideas/clues on what they might be

 

[Daydream believer]

Float off the end of the arm of the filler sensor

 

[Peter Morgan]

No, float intact

 

[Refueler]

Float off the end of the arm of the filler sensor

That's what first crossed my mind ... but he said rubbery ... most floats are plain plastic.

Maybe someone in the past has had a brainstorm of putting in balls to keep from having crud settling out ? If you use Enzyme based Bio-Cide - that will allow the results to pass through filters and be burnt. Just a thought ?