I would basically keep an eye on weight and physical condition. I would check to see how the droppings are after he eats, If you keep him in a separate tank you can monitor how much he eats. Also, using a damp paper towel on the bottom makes it easy to monitor droppings. If he has runny droppings see the bottom of this post.

Whites Tree Frogs are active at night and will look sleepy during the day light hours. I would mist twice daily at a minimum to see if that changes behavior. The substrate should feel slightly damp on top and not dry.

If you are finding droppings from both frogs, usually found both in the water or below where they sleep, then you know they are eating. You can also look into having a fecal exam done with a local herp vet to get a clean bill of health on the frogs.
You can monitor how much they eat by using a glass cereal bowl. Place the bowl down into the substrate and drop the crickets in. A round bottom glass cereal bowl gives the crickets zero traction to jump out and escaped crickets sooner or later fall back in.

Now, as Dalton was getting to, hydrometers are not the most accurate things and when I select one I try to compare the readings off a few different ones to find several (different brands) that are reading the same. Basically, if your glass is wet, soil damp, and part of your top is covered, you have humidity.
If you have any type of lighting for daytime, this will burn off humidity quickly. Just keep an eye on it to see how quickly your readings change between misting. Your WTFs like between 50 and 70 percent and night temps around 68 - 70, days between 75 - 80.

One thing I would like to also point out is that your frogs like to sleep and perch on plants and branches so have enough up high for them to be on and sleep on. Snake Plant and Bromeliads are great for tree frogs and they usually are more active up high when they have paths to move across. IE: branch to leaf and so on. Mine slept most of the time in the Bromeliads and one loved to sleep on the snake plants leafs.

Lastly, I'm not a fan on treating something unless I know it is in fact the problem. A vet is your best bet for making that determination:
It is possible your frog has the following condition:
Symptoms: Listless, often found in water or hiding, watery droppings.
Cause: Parasites in intestines, “Protozoa”
Treatment: Parasites are always present in the intestines, but they will cause problems when the animal is weak. Treat with METRONIDAZOLE (FLAGYL) 1 drop in 8 drops of tepid water. Place one drop on the back of frog. Duration: Each day for 6 days. Treatment for the vivarium: 1ml per 5 liters of water; use this to spray the vivarium.


Keep us posted.