Yep, you can treat for algae, even for asphaltene. (hard to see what it looks like right now with the glare coming into my truck).There's an easy triage here... eliminate algae by treating for it. Go to the local marina and get some algaecide - they have severe problems with it, and I'm told that the stuff they use is 100% more effective than anything you can get through the trucking industry. If the problem goes away... bingo!
If soot is getting into the returned fuel, you have a vastly more expensive problem.
So huck the water bottle in a ziplock baggie, scribble the lab address in black sharpie and send it off?
That will prevent getting an analysis of Carbon, water and traces of urine for some reason.You should use a sample kit to keep from contaminating it.