I have a 75-gallon tank and clean out about 10 gallons every week. Since yours is an 18 gallon tank, I would start with cleaning it every two weeks by siphoning the substrate and removing and replacing 7 gallons.
If your water looks dirty try changing 5 gallons every week. But I think you will be good with the two week option.
If you arent going to do regular maintenance, keep it to 5 or less godlfish.
every other week
Once a week :))))) i have experience
Every Second! Gold Fish are Very messy and need a filter to keep them alive. I suggest you put your Goldfish in a pond. Make sure there is more than one, Goldfish have friends just like you and me. Or you could get a 20 gallon tank (the tank has to be big enough so goldfish can grow up to 10 inches long WITH a filter. okay long story short, one inch per fish needs at least a gallon!
every two weeks then change the water every four
Not neccessarilly. You probably need to change its water more often and if possible get a fish tank (aquarium) to put it in. Bowls are not the best containers for goldfish.
Most common Goldfish are pond fish and are not really suited to life in an aquarium. They need to get up speed in their mating chases and they jump out of the water as they spawn. I guess if your tank (Aquarium) was big enough for them to do this they could spawn in there. Slower, poor swimming types of fancy goldfish like Black Moores and Lionheads etc are often spawned in aquaria but these tanks are usually a minimum size of 6 foot x 2ft x 2ft.
The main thing you need to do is do some research. There is a lot you need to know to have a successful aquarium. There is such things as knowing the following:What size aquariumNitrogen CycleWhat fish are compatible with each otherHow often to feed your fishHow to clean your tankHow often to clean ouyr tankAnd there is much more.Take the time to learn about building and maintaining an aquarium before your start and you will be successful.
-A bowl or aquarium (it will greatly benefit from a larger environment to grow in). -More goldfish for company. -Fish food, tetrafin, or wardley's, flake food or crumbs. -If in an aquarium an airator is good. -You will benefit from a filter so you don't need to clean the tank as often. -Water conditioner to make tap water safe. -Tank ornaments such as gravel, fake plants, water plants or decorative hiding places. All things on this list can be purchased at a pet store or in the pet section of Walmart.
weekly 10 - 20% water changes
you should do a 50% water change every other week
You feed it the same amount as any other goldfish which is whatever it can eat in five minutes once a day. Be sure to remove any uneaten food you can to keep the water clean.