You need to use a biocide to keep the bacterial and fungal growth down if you're storing diesel for a long time. I use Biobor
and have for 15 years. It works very well and will prevent growth in your tank. Killum is also very good, but very strong. It leaves a residue in the filters and creates a lot of debris. I like Biobor better. Long term antioxidents are good too, but the biocide is the most important additive.