Primary Filter
 

I was noticing the little primary filter is very dark looking inside with the fuel. Should it be more clear looking with the diesel fuel? I know the ones with gas are quite clear to the eye. Would it be maybe, that is it is ready for a change out?

Clean it or replace it and see if it darken up again before the end of the Month. If it does you have something growing in your Fuel Tank that may need to be killed.

After about 7 years of use I have some black flecks of stuff in mine that I am keeping an eye on.

Another potential source of dark fuel is degrading fuel hoses. My rear tank outlet host was dissolving from age mixed with B99 use. It turned the fuel black. After replacing it and running a few tanks of fuel it's clean now.

Could be.
I replaced My Tank Hoses 6 Years ago; cut the metal Ferrel/Collar off of the old Hose so I could use clamps on it.

Fuel filters are cheap and easy to change. When in doubt, change them out.

What are the latest thoughts on a good biocide to use?

Is there any issues about breaking prime on replacing just the primary filter alone??

Do it with a full tank and the nose pointed down hill and there won't be.

Be sure to pinch the hose coming from the tank before removing it from the primary filter or you will spill a lot fuel, especially with a full tank and nose pointed down.

I still have most of a bottle of biobor. I used some in an earlier diesel. It is approximately 3+ years old. Would it still be ok, and would I want to check for water bottoms before using?

Microbes don't grow without water.