I wouldn't treat with Fungus Clear unless you notice sick fish.
That's the thing, I did have a fish in there before that had a fungus on it (Neon Tetra). I think when I bought the fish it was already sick, so I had to return it to the store for store credit as it only lasted two days in my tank before developing the fungus. It was removed and taken back to the LFS, but I don't know if I should still treat my water for fungus even though the fish that had it isn't there any more.
Also, even though I have a twenty gallon tank, I KNOW that it doesn't have 20 gallons of water. More like 15 gallons or so (I honestly am not sure, I'm guessing about 15 gallons). The rule of thumb is that I can't have more than 15 inches of fish in that tank. Is it okay to have more fish if I have good filtration?
A couple more questions that I just remembered about...
1. On a power filter that rates from 20 - 40, the low setting is for 20, middle for 30 and high for 40, right? It gives no indication on the instruction manual nor on the box.
2. Should I invest in an air pump for this type of aquarium? Maybe if I want to put more fish in there?

