Originally Posted by RigglePLC
The customer is always right in this situation. Do NOT yourself claim that it is not possible, say you don't know. DO show him the manufacturers response. Ask the customer what he wants you to do. Compensate the customer in some way--say a free ap next year. Its a cost of doing business. If you handle this right, professionally, he will give you referrals, if not, he will tell all his friends that you are incompetent.
Actually you are right--it is highly unlikely. Fish die all the time, too many fish for the size tank, warm water conditions, low oxygen, disease, and sprays of pyrithroid products (as pointed out above). Why would the water get clear? Hmmm...maybe they used an algicide...and it killed the fish. Swimming pool algicides probably do not belong here.
That is absolutely the right way to handle things! My response about Fiesta not killing the fish was purely to help ecoguy sleep at night knowing that he didn't inadvertently kill anyone's fish. I'd never suggest telling a customer they're full of crap, regardless of whether or not they actually are.
Not having a manufacturer's official response, the most you can say is "duh, sorry". With it, you can claim that you try hard to be responsible, you use due care in what when and where you apply, and did your best to not harm their pets. And you can back that up with a manufacturer's statement, and then ask what you can do to make them happy.
I agree that the water getting clear suddenly is a warning. Copper based algicides would certainly have killed the fish. But unless you're prepared to take on aquaculture for this customer, I wouldn't suggest you provide advice in this direction (and certainly do NOT cast blame). Maybe spread some doubt, but tread lightly.