Diesel in coolant

I replaced the injector cups after i realized that there was diesel in my reservoir. I did a very thorough rinse out with a regular water and a bunch of distilled water. I have read that sometimes teh injector cups wont secure properly and can still leak.

My question is how can you tell whether its lingering diesel fuel or if you still have a leak? I removed the coolant and let it sit over night and this is what i found. it is a very thin layer

I have been checking the reservoir to see any major separation from the diesel and coolant, but there has been nothing noticeable. I ran it on distilled water for a bit, and noticed nothing in reservoir

Do yall think its too much and I ll have to do injector cups again, or is it lingering fuel thatll never come out?

Thanks

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