Fish do have tongues. It could have been a parasite or low levels of oxygen in the tank. Wrasses and tangs need high oxygen levels to remain healthy. I used to lose wrasses all the time until I set my powerheads so that they caused a lot of ripples in the surface of the water. Now I do not have any problems with wrasses.
A rubbermaid trashcan or two would work. One for the rocks and one for the fish. Depending on how you get it done, it can probably take a few hours or a few days. If you get the overflow put in using silicon you have to wait 24 hours for it to cure.
I think it is definitely worth it to do. I don't really trust HOB overflows. All it takes is a power outage or a slipped hose and your house is soaked.