Platy, take whatever advice you want, but goldfish are cold water fish. If you poke around any cold water / goldfish forum, you will never see a heater being recommended for the fish's long term health.
No offense to pet/fish store personnel, but generally they don't give the best advice for long term fish keeping care. They're there to sell you products.
You also need to have your own water parameter test kit to be a member at this forum (I'm KIDDING!). Seriously though, you should have one. Read up on nitrogen cycle/cycling/beneficial bacteria as it relates to fish keeping. Good luck! Nice looking tank and fish!
Joel