A good general rule is 1 gallon per goldfish. The larger the bowl the longer it will take for ammonia to increase to harmful levels and the less often you will need to clean it.
The essentials are:
Aquarium salt is highly recommended for preventing diseases with goldfish. Plants and other decorations are up to each customer.
A filter is not needed, but using an air driven filter is an inexpensive way to increase the time between water changes. The goldfish will also benefit from the increased aeration. A heater is not needed.
It is very difficult to starve your fish, but very easy to overfeed it. If the food falls to the bottom of bowl you are overfeeding. In a bowl if you overfeed 2 or 3 times in a week you could kill your fish. We recommend you feed your fish every other day once a day. Pellet food will float longer than flakes but either type will work.
A goldfish bowl without a filter should be cleaned twice a week. With a filter you can clean it every 7-10 days.
Remove 1/3 of the water and put it in a different container. Catch the fish and put them in this water.
Pour out the rest of the water and refill the bowl with clean water. Refill the bowl with some water and “swirl” the gravel with your hand and dump out this water. You are using this water to work out the gravel. Repeat this until the water seems clean.
Refill the bowl ½ full with water that feels the same temperature as the saved water add dechlorinator and then poor the water with the fish back into the bowl. Top the bowl off with more water if needed.
Add aquarium salt if applicable.
If your bowl is extremely dirty you may need to change all the water.