Help My Fire Belly Toads are dying

[PAFishMan]

For Christmas I got my daughter 4 Fire Belly Toads. I have a tank set up with a gravel and swimming area and a mister. I put a heating pad for reptiles on the bottom of the tank under the gravel. There is a under gravel filter in there as well. Looks like a set up that would be perfect for frogs. I checked the temp and it's 72. At first the frogs were happy. We feed them once a day and they were eating crickets every time we put some in. They were singing a couple of times a day that we noticed. Sometimes they would sing when someone went near the tank. I used distilled water to put in the pond area. About 4 days ago we noticed one in the water with it's legs stretched out. It did looked fat. I was sure it was dead and got it out of there. Then last night I noticed another one dead the same way. We noticed just under a week ago they stopped signing and just before the first one died they stopped eating. I just tried to feed the two we have left and the crickets just walk over them and they don't go after them at all like they use to. So now they won't eat. These two that are left look skinny. Does anyone know what's going on? Should I check the water like I do the fish tank? What values am I looking for? It's distilled water so I don't think it went bad that quick with a filter in there. It's only been up 3 weeks. Are crickets going behind the wall and dying therefore making the water bad? I did try to feed them meal worms from the pet store but they didn't eat them. A day later we did smell something rotten from the tank, I thought the water was bad but it was the dead meal worms in the dish so I got them out of there and the smell went away. Hope I can get some help before I loose the lat two. Oh, at night I do set the temp in the house to 68 but I don't think the water in their pond gets that low. Thanks
PS: See photos, the frog on the lily pad is not real it's a fake one that lights up different colors that hasn't worked since the batteries died. The two that are alive are in each back corner of the tank. Thanks