Heater for my fish tank?

Your fish are fine, that's a good temp for goldfish. Save your money for a bigger tank in the future when your fish out grow your tank.

You don't need a heater- goldfish are coldwater fish. Raising the temperature would stress them and make it difficult for them to breath. Warm water contains less oxygen. The temperature is really the least of your worries.
Your tank is severely overstocked. The goldfish may be small, but even a one small goldfish produces a lot of waste, and you have six squeezed into a tiny ten gallon.

Do you have fancy goldfish or commons? If they are fancy you should have at least a 75 gallon tank. If they are common goldfish than they shouldn't be kept in a tank at all.

It's not just about room-it's about dilution. Even with double the filtration (which you should have) ten gallons cannot produce enough beneficial bacteria to handle waste from six goldfish. You need the room to dilute their waste. Otherwise ammonia and nitrates will build up in your tank causing stress, sickness, and probably death. The goldfish in your tank may stay only 2 1/2 inches for as long as he lives in your tank. Nitrates inhibit growth. If a fish cannot grow, then they are not thriving, just surviving.

~If they were cheap (less than a dollar) than you bought common goldfish. They grow up to a foot in length and can live 12-20 years when given correct conditions. Because of their immense size they should only be kept as pond fish.