Well I guess my character reference email of you and your most excellent catering care of your fish would be superfluous at this point. At least you get a 50% discount out of it. I will send the reference email if you like. It might further inflame the owner, but I can make it very noninflammatory and thank him for giving you a discount.
I had a unpleasant experience with the owner our local fish shop last week. Really riled me. This was day I was going to get Horseman Cats.
Bear in mind I have been going to this shop for many years. I have seen many employees come and go. A new assistant got pair of Long Finned Cherry Barbs, and a Yo-yo loach for me. I was going to go for the Horseman Catfish, when suddenly the owner who had been nearby, reprimanded me for acquiring the loach from a tank which wasn't marked. I apologized that I didn't know. (The employee had okayed it.) I paid for the fish and left. Another day, I returned to buy the Horseman Cories and I brought it up to the manager, (owner wasn't there). Manager apologized and said that wasn't right and said that she would get fish out of that tank anytime I want.
I told her no, but thank you, I would respect his (owner's) wishes and walk past that tank with blinders and tunnel vision (maybe a backward peek, over exaggerated leer).
The tank should have been marked--not for sale. And the employees instructed thusly. I had gone home feeling terrible, like I had done something wrong.
The manager and another employee made me feel much better. One said the owner was having a bad day that day and he gets that way. Shoot, I have spent well into 4 figures, maybe close to 5 over the years. I feel like an investor at this point. If he had stocks, I would be a stockholder and on the board of directors and I would demand "need more Piranhas around her-- I guess we have enough around the table here."
Please don't feel bad. I think you were appropriate in what you told the fish store owner. Some fish stores won't give any type of refund for a dead fish. I had to threaten small claims court when an expensive fish died and a (different) fish store manager wouldn't refund or store credit me. I would freeze your dead fish and bring it back, unless the owner doesn't require it.
Sometimes fish just die. It might have had a nonapparent illness before you purchased it. And some wild caught fish can be susceptible to captive bred fish pathogens when exposed to them.