What can be done can be undone, when risk-adverse, compliance-oriented bureaucrats are sufficiently motivated. An exemption is just as easy to create as the original rule.
"If the government keeps the rule, hes worried that farmers will be forced to circumvent the rule by reducing the size of their tanks, which would lead to farmers spending more money to refill the smaller tanks more frequently." Source
Smaller tanks: "Check...switch to coal ...Check ...."