20 gallons will be fine for about 3 or 4 ACFs. ACFs are quite different than ADFs. 1) they are much larger; and 2) most likely will eat other fish (although, there are differing opinions), and 3) could tolerate lower temperatures. You made a good choice
I recommend having a filter, however, they will do nicely without one if you do weekly water changes. They are sensitive to high flow filters. These frogs have a well developed lateral line system (like fish) that detects prey through water movements and electrical impulses. High flow filters cause waves that may confuse the frog.
I do not use a heater and keep my frogs at room temperature. The optimum temperature is between 69-71(F). Stress and death may occur if the temperature is above 86(F).
Freeze dried bloodworms are a no-no. Try live or frozen. Freeze-dried bloodworms may cause bloating. They can also take Tetra's Repto-Min. I feed all my frogs Reto-Min and have not experienced any problems.
Water quality is the most important element in the care of ACFs. Always condition the water, I use Stress-Coat (1 ml per gallon). When doing water changes, make sure the temperature of the new water matches that of the tank, ACFs are sensitive to temperature changes. I have been keeping these frogs for 30 years and really enjoy them.
I have noticed from the posts on the forum that diseases are really affecting ACFs. It could be over-breeding or just poor habitat conditions. The two major problems seem to be bacterial infections and bloating. To prevent disease, try a product called Lymnozyme, it is a pro-biotic (not a medicine). Pet shops are not always reliable when it comes to caring for frogs. Check out some of the posts in the aquatic frog section, they will be quite helpful in getting you started with these fantastic frogs (yes, a bit biased)