That's awful! It was Ammonia-0, Nitrite-0 and some Nitrate (kept as low as possible)? Was anything different with your PH? Maybe you should tear down the tank and clean everything well to get rid of anything that might infect new ADFs then recycle it. Id just hate for you to lose new ones to the same thing.

On a happier note what kind of frogs are you going to get, more ADFs or planning for something else?