It wasn't just shrimp, it was fish dying left and right. Otherwise I'd say it was for sure the copper because the house was from the 50s.If you were using a dechlorinator that doesn't also remove ammonia (a byproduct of the chemical reaction caused by dechlorinators) and/or doesn't remove chloramines, you may have your culprit. Another possibility is aging copper pipes. Copper and shrimp are no bueno. Were you on base? Military bases have a long and sordid history of supplying bases with water that is not actually safe to drink.
It probably was the chloramine. Regardless it makes me nervous.