I am sorry to hear about your frog.
It sounds to me like your tank needs to properly cycle. Taking ALL of the water out of the tank ruins the cycle. When you do normal water changes, you should actually only do a partial change, not take out all the water. Your tank needs time to build up the beneficial bacteria to keep the tank cycled and cause your aquatic pets urine to not poison them. When you take out ALL the water, it will cause your tank cycle to break and start over again which could be fatal to the creatures that live there. You need to get the tank cycling and the good bacteria building, which cannot happen if you take out all the water. I am sorry to say but.. if a tank is properly cycled a bit of leftover food would not normally outright kill the inhabitants..
It can also be really stressful to fish/frogs to be taken out of their tank when you change it out, so ussually only partial water changes are done.
Did you read the links given to you about cycling your tank? What kind of dechlorinator are you using?
For now, I would say you will need to do daily PARTIAL water changes to get the cycle up and running. Use a grael vacum to siphone up leftover food, and only change out 40-50% a day.
Good luck!