Thanks Kevin
I learned my lesson, the amount - and position- of filters as you described is what I am adding 'as we speak'.

I get my Jet A-1 also from the local airport, from the tank trucks they use for aircraft refueling. To be honest I think I messed up after that when mixing, or pouring it over from one can to another. (I don't do that anymore, this was some left-over from when I ran it on a test stand)

Anyway, apparently it's not algae, back to cleaning and adding filters......