Leopard bullfrog tadpoles are dying..help?

I collected many tadpoles with my son at a lake that is being drained here. We gathered tree frogs, toads and bullfrogs. Everything has grown and morphed except the bullfrogs. It has been 6 months! They have been identified positively as leopard (spotted) bull frogs. Yesterday I found that a minnow expired in their "true" to life lake environment the remaining three have been removed to a smaller environment with clean water and fresh food also without dirt and instead only river pebbles so that i can keep a closer eye on them. they are almost thumb size and 2 have small legs. They barely move/interact/eat at this point. So I need major help I don't want them to die but I have no local ponds to return them too, I am trying all I can but I am afraid they will die like the 4 others i had of their species and I don't know what i am doing wrong... yes i am treating the water and letting it sit out for at least 48 hours after treatment before using on them; until today their water was pond water (70 percent) The other breeds did fine but these didn't. I am thinking the crayfish that was in the tank and went missing died and made their water fetid and that's why their siblings died before i understood and removed them. If so how can i help them get well ? my son and I must admit me too are very attached to them. They used to swim to me and seemed to actually enjoy attention and I do not want them to expire esp before morphing I have devoted 6 months to these frogs and I wanna see them become frogs can anyone help???