Listen man, I have Oscar's as well. I worried terribly about this and guess what I found to be the problem? GROWTH. Your Oscar is growing, the white fin ends are merely the undeveloped fins that will eventually grow into healthy bigger ones. Now my youngest Oscar's are around a year or so old, and the white tipped fins have drastically slowed down. So give it a little time, continue the good caretaking you are giveing it, and you'll see, one day he's gonna be HUGE!
That 20G has got to go though, Before you know it he will outgrow that tank and definetly become sick as a result of having such a small environment. it is imperitive that he gets a bigger home probably within the next month or two, you will see. Good Luck.