ok here are a few problems :
with no filter and such a small tank - changing the water only once a month means you are leaving them in high levels of ammonia (frog waste) for a LONG period of time and this is very very bad for them!
for a small tank like yours, with out a filter, you should be either doing a daily small water change or a 100% water change every 5-7 days. (Think - fresh water animals liked "FRESH" water)
Once a month is no where near sufficient. Ammonia literally burns their skin and will eventually cause Red leg - an fatal disease caused by poor water quality.
you should also be treating your water - yes even "fresh water" because tap water has chemicals in it. you need a dechlorinator at the very least.
Food - - bloodworms should be fed only as a treat (once a week or so)
As soon as possible - pick up either HBH frog and tadpole bites or Reptomin sticks (these float and need to be broken up for the frog to eat)
In summary - - change their diet, clean their water more frequently, treat their water, get a heater and consider an upgrade.
Larger quantities of water are more stable which helps prevent illness.