Personally, I think if you'd have handled the situation a little more delicately, he'd have been more cooperative.
Example: I went to my local Wal-Mart back during the days of the betta/vase fad (shudders at the mentioning of it) and I saw a similar situation. The bettas were in cups that wouldn't hold more than a metric cup of water... and they were half full of yellowish water that was swirling with half-rotted feces and fungus. Some of the bettas were floating on their sides and a few were dead and had obviously been so for a few days from the level of decay.
I went to the next nearest department, flagged down a worker there (because they don't staff directly in the pet department. Usually the fabric dept next to it took care of the fish), and asked her about how often they changed the water. She told me every other day. I asked if she knew that there were dead and sick fish in the cups and she tried telling me the same story. I simply asked her to come with me and make sure. I held up a cup with a fish on it's side and proceeded to explain dropsy to her. She listened with half an ear until I held up one of the dead fish. Even she agreed it looked like it had been dead for more that a few hours.
I told her I didn't mean to make her job any harder, but that it wasn't fair to the fish to treat them like that and that no customer would want to buy a fish that was surrounded by dead and dying fish. I told her I intended to speak with the management there about all the workers taking better care of the department. She said she wasn't really busy at the moment with the fabric department... I even offered to watch it for her in case anyone went in looking to buy fabric. And so I stood there for about 30 minutes while she changed the water and threw out the dead fish and treated the sick ones the best we could with the available treatments.
She thanked me for my help and then I went and talked to the manager, strongly commended her helping me, and then filed a complaint about the previous treatment of not only the bettas, but all the other fish. It took me a few months of persistent, honey-tongued diligence... but they eventually started medicating their tanks, removing dead fish and changing their betta cups every other day with treated water 3/4 full each time. It wasn't always perfect... there were hitches to the system (they have to specifically tally the dead fish because big corporations have to write off the losses and show they made no profit from the dead fish at the end of the week or something, hence why they usually leave them in the tanks, seeing as how there's no other place to keep them without them reeking. Most people don't know that.)
But we came to some nice compromises by me being nice, even though there were times when I was utterly outraged at them. Sure, not all situations end so nicely and not all of the workers I came across were pleasant or helpful in the least... some could be downright obnoxious. But I knew if I went in there and blew up, I'd build walls instead of making bridges, if you will.
As they say... you catch more flies with honey than with vinegar. ^__^;;
