Hi Yuri
Dropsy seems to be a common problem among aquatic frogs. It is caused by an osmotic imbalance - the ability for the body to take on excess water but not able to release it. The problem is often associated with infections or kidney disease. It is a serious condition but easily treated.
The most common treatment is a salt bath. Place frogs in a sterilized tank, add about 5 milliliters (one teaspoon) of non-iodized salts (sea salts or Epsom salts) per 4 liters (one gallon) of dechlorinated or spring water. Place the frog in the prepared water for 30 minutes. Repeat daily (with new water) until the bloating clears. This should work. Do not feed these frogs any freeze-dried food as this is known to cause bloating.
Good luck and let me know how the frogs are doing.
By the way, dropsy is a term that was commonly used in older amphibian literature, today it is referred to as coelomic distention.