Your ammonia would probably spike from the dead fish so their test doesn't prove anything really. It's a short sighted approach - they lost your business. I'd fish die over night and LFS put it all on me too. Whereas I followed their advice with set up and stocking and ammonia spike was because my tank wasn't cycled.
especially first thing in the morning. They bicker and chase for a few minutes until they decide who's boss for the day. I considered Congo tetras but I worry their fins may be a little long?

