Agree with Dave that 1 gallon is too much. However, from an aquatic tox issue, it could have been a much worse chemical. Cumene ,also known as isopropyl benzene, was one of those chems that recently went through the reregistration process via EPA, including tox studies on fresh water animals. Cumene actually falls under the moderately toxic category under rigid lab conditions, but is probably much less a hazard in open water. It has very high volatility and biodegradability in open exposure situations. I'd be more worried about people exposure in the area where it's "flashing off" than potential fish kills.

Like I said, not justifying any of this and certainly wouldn't want it in my drinking water , just saying it could have been a much worse scenario from a fishes point of view.