Agreed with Hans, goldfish don't belong in your tank. They are temperate fish, not tropical... and they produce a lot of waste. For 2 fully grown goldfish, I typically recommend a 55 gallon tank with lots of filtration. I'd also look at moving your pleco to a larger tank as well... that will outgrow your tank.
Other than that, increase your water changes, and reduce your feedings, and that should help lower your nitrate levels. My tanks usually got two 50% water changes in a row at least once every 1-2 months which helps bring nitrate levels down to undetectable levels. Then from there, I do 50% water changes once a week. My FO tanks rarely have nitrate levels over 10ppm... but then again, I feed lightly, and my tanks are very lightly stocked.
HTH
-Richer
