My first question would be how long the 3" frogfish was in the tank. Considering that many retail facilities quarantine fish in tanks much smaller then 24 gallons for extended periods I think it is safe to assume that a small amount of time in a less then recommended tank can fare alright. Many LFS
's utilize long slender tanks with acrylic partitions, only giving the fish a tiny amount of room to house the fish that are offered for sale; even in those tight quarters fish can survive.
While I am totally not advocating keeping fish in tanks too small for them; or tanks too small to accommodate them as adults; I don't believe that the stress of a 3" fish being in a 24 gallon aquarium would kill a frogfish. A surgeonfish perhaps or even another species that is an active open water swimmer. Famous Discus breeders not only house but successfully breed Discus fish in 20 gallon long tanks. Tanks of 55 gallons or more are often recommended for Discus fish but the nature of the fish's swimming habits can make them easy to house in smaller quarters.
Another question I have for Bonez is how often the fish was fed and what it was eating. Also how frequent were you doing partial water changes. Bio-load could have played a role in this fish's demise although if PWC
's were done on the proper basis (maybe even more frequent then recommended) it may not have been an issue. Also a post of water chemistry readings:
would be very helpful.