My frogs died

If the tap water wasn't dechlorinated, it is possible they soaked up some chlorine or other heavy metals from the water since frogs are more sensitive to that sort of thing than fish are. Also, feeding them flakes, I've found frogs don't do too well on a diet of flakes, it doesn't sustain them well. There are some frog pellet foods on the market that contain more nutrients that frogs need and most frogs, even if they've never eaten them before will eat them because they contain a strong smell that definitely is more appealing than flakes. This food sinks to the bottom which is also more appealing as frogs eat more from the bottom of the tank than the top.