If your 55 gallon aquarium has standard sized filter intakes, chances are the fry will not survive anyway. If you are trying to save the young, you may want to move them back into the 20.
ammonia is typically a short term problem or long term if you do not have adequate biological filtration. If this is a new problem with the 20 gallon, as a result of a recent change in filtration, etc., it may be only temporary and can be kept under control by your water changes. You will often read that ammonia should read 0. This is ideal, but sometimes, not always possible. The actual toxicity of ammonia can be calculated more accurately when comparing the results of your test kit with water temperature and
pH. I will try to find this information and post it here for you.