Can you post more specs on the tank, e.g. age, temperature, etc? How long did you have the fish before they died?
Did the fish die suddenly or over a period of time? Did they show any signs of stress, eg gasping, ick? Any chance anyone put something in there?
Organic toxins (like cleaners, insecticides, etc) can take out fish and leave inverts unharmed; heavy metals often take out the inverts. However, I have seen major die offs like this from things like temperature or pH fluctuations, low alk, etc., and indeed stray voltage.
In mystery cases such as these, big water changes are the first line of defense. I am surprised the LFS didn't suggest this, and went for Iodine instead. Rather silly, IMO.
Definitely not all due to the mantis!