The ULSD supposedly has a shelf life of about 2 months. We let the bus sit all winter long without treating the diesel tank, and it grew a phenomenal amount of algae. It was so bad that it would plug a filter after only 10 or so miles. The stuff I drained out looked like green apple cider. Right now we've dropped the tank to steam clean it, and are putting new fuel lines on. If you're going to let it sit, put some kind of biocide in it.
'01 Dodge Ram 2500, Cummins HO, 4x4, 6 speed, homemade WVO system
'96 International/Bluebird school bus, DT466, homemade WVO system, www.thebiggreenbus.org
'04 Jetta TDI, Greasecar WVO conversion (chases the bus around the country)
'89 MCI 102A3, Detroit 8V92TA, Frybrid/Rover Hybrids custom system