I agree.. goldfish need a large tank of at least 30 gallons and about 15 gallons per fish. Although they are doing ok in your tank their growth is being stunted by keeping them there... I had a goldfish in a 5 gallon tank for 5 years before i knew anybetter and when i put it in my aunts outdoor pond it grew to 6" in no time.
Also as Fishyfanatic said, goldfish are coldwater and your other fish are tropical.. Gold fish prefer a temper more in the low 70's whereas tropicals usually like it kept about 75 (i keep my tank at 80)
Your tank looks beautiful, but if you want to add a "centerpiece" fish, you might like a male flame dwarf gourami, they only get about 2" and they are very colorful.