What kind of goldfish is he? If he is a fancy, you can get him a friend in that 30. You can also have a school of white clouds with the fancy golds. <That would look almost like a school of livebearers ....> Commons will prob have them for snacks .... still a bit risky with a big fancy, but if the WC
's are at least 1" should be Ok.
If you have a comet or other single tailed goldfish, you would want to look for a pond. You can prob keep it in a 30 until ~5-6", but they do best in at least a 6' tank.
Yes, goldies definitely need good filters ... most fanciers have filters that are rated at least double the tank size. <A small canister would not be out of place for your 30 .... I run a HOT Magnum in my 10 as QT!>
A heater is good insurance if you live in cold climes. Fancies not just "tolerate" warmer temps, they are sub-tropicals, & need to be in temp of mid 70's. Since you are in Sudbury, you would want to keep the heater for winter. Just set it to 72 or so year round.