In general, diesel can still be usable at 6 to 12 months of storage provided that measures have been implemented to ensure its good quality.
It should be noted that water/moisture is a common enemy of metal tanks and diesel fuel. However, algae is not the only consequence of water; rust can develop from water, too, which can build up inside a metal tank, compromising diesel quality and potentially causing damage to your engine.
Water may also occur because of condensation when water vapor from warm air encounters a colder surface. Metal tanks may be subject to this phenomenon especially when it’s scorching hot outside.
To reiterate, even though diesel can be stored for a fairly long period of time, it does need to be stored properly.
Coerco's poly diesel storage and cartage tanks directly address the above issues, making them ideal for farm use. Poly is 100% resistant to rust/corrosion and isn't prone to condensation because it doesn't attract moisture from the air. These superior poly diesel storage and transport solutions give you the best chance of having a viable diesel after months of storage.
See here for more info on how low quality diesel tanks could create problems for your engine: