Hi the thermometer should be right under the heat lamp, remember they are aboreal, so the floor temperature doesn't matter too much. You want to use a digital one, then place it on a branch directly under the bulb, whilst the analogue thermometer can measure the ambient temperature. 10-gallon tanks are really too small for tree frogs, despite what others may say. It's also very easy to overheat them. In larger tanks a better thermal gradient can be created and more space for the frog to live. I think the frog could have perished due to too much heat and not enough space to get away from it. I have two and keep them in one of the 18" cubed exo terras and this is probably the minimum size tank I would keep them in but you can easily fit another two in this size of tank (18" cube)