Are these fan tailed goldfish (fancy goldfish) or standard comets (feeder goldfish)?
30 gallons is only going to buy you a little bit of time. Comet goldfish grow to 14 inches long, and fancy goldfish average about 8 inches. Minimum tank size for an adult fancy goldfish is about 90 gallons for 1 - 2 fish, and for comets, 125 gallons or more for 1 - 2 fish.
Healthy goldfish grow very fast... to go from 2 inches to 6 inches within 1 yr is easy and should be expected.
Fancy goldfish need to eat sinking pellet food so they don't gulp a lot of air, as it causes swim bladder problems that can lead to an early death of the fish. Comets are fine with flakes until they get larger, then you can feed them koi pellet food to keep them going strong.
With a bowl and no filter that water would have to be changed at least once/day every day. Their temp should be kept as cool as possible (65 - 68 degrees is optimal). The warmer the water the less oxygen is in it, so changing part of the water a few times/day (a cupful or 2 at a time) would help a lot until it's in a filtered aquarium where it belongs.
Goldfish can live 25 (fancy) to 75 (comet) to 100+ (koi) yrs, so this should be expected to be a very long term committment.
If this seems a bit overwhelming, you might want to consider taking the fish back and exchanging them for something more suited to your needs. A betta makes a wonderful bowl pet if it's properly taken care of, which isn't hard to do. A small tank would hold a few tropical fishes that would also be easy to care for.
If you need help/suggestions, please let us know!