I wouldn't rely on what they're telling you. If you want to keep a pond you really need to buy the test
kits and learn how to do it yourself. It's really very simple and gives you the ability to test when you need
to.
There's not much you can do about it now but for the future, never put new fish in with existing ones unless
you're prepared to lose them all. New fish need to be quarantined for several weeks minimum before ever being
introduced to the pond and your existing fish. Like you said I would suspect that the new fish started the downward
spiral and the overpopulation and sand at the bottom, like Mary said, just created the perfect storm scenario.