Fish aren't really a natural diet for them so I'd stop feeding those for now (just in case it's related).
Are the crickets dusted with calcium powder or any other supplements? Temperature seems good. If possible, I would test the water for ammonia since it could be poisoning. Pet stores will test water samples for free. You've probably already done this but I'd give the tank a good cleaning. Firebelly toads are poisonous and the toxins can supposedly build up in closed systems. They can basically poison themselves.