Actually using hyperosmotic ARS can help remove excess fluids. I keep two containers on hand at work one with 5 gallons of standard ARS and one that is about 10% more concentrated. The hyperosmotic is usually used for several hours at a time as opposed to te 24/7 with the standard. I doubt that the little bit of condensation seen in the container is going to be a big enough change in the osmolality in this case.
If you can get a vet to do it removing some of the excess fluid in the abdominal cavity can give some relief to the frogs and it can be checked for the presence of bacteria.
I really suggest consulting with a vet on this as metronidazole may not be the drug of choice. If you want you can contact Dr Kevin Wright (author of Amphibian Medicine and Captive Husbandry through this site
Arizona Exotic Animal Hospital for a consult) or Dr. Frye who I think was mentioned earlier in this thread.
Ed
