Lost baby fire belly toads

[cwebster]

Purchased four eight week old fire bellies two weeks ago. Suddenly one died (was on his back). Not sure what I did wrong. They are in a 10 gallon semi aquatic tank, with lots of places to climb out and with a filter and overhead light. They have been very active, happily eating hydei fruit flies and pinheads, dusted with calcium powder every other day and vitamin powder once per week. Am using bottled mineral water, changed weekly and with any leftover feeders removed daily. House temperature is 72. What am I doing wrong? Am so sad and devastated.