Goldfish are a TON of work. Also several hundred dollars worth of investment to keep them alive for their potential lifespan. (Cheaper if you want a bowl and let them die every week and replace them like an inanimate decoration)

You should have 10 gallons per 3 inches of goldfish. They produce a ton of ammonia and ammonia will kill them.

The filter DOES NOT suck up all of the poop and debris. Most of it will end up in the gravel and you will buy an inexpensive gravel vac an vaccuum the gravel when you do the water changes.

The filter DOES provide a place for your important bacteria to grow. This bacteria will come about within a few weeks of starting the tank and will neutralize the ammonia and nitrites. In the beginning before the bacteria grows in the filter, the ammonia and nitrites will need to be controlled by you and could need daily water changes depending on how many fish you have. You will need an expensive but vital water test kit for ammonia, nitrites and nitrates. Test the water daily until its cycled. Cycyled means the beneficial bacteria has grown and is controlling the bad chemicals. Once that is done you can change the water every 2 weeks or as needed. With goldfish its more like 20% weekly in a healthy tank.

DO NOT buy a product that says it will instantly cycle the tank unless it is kept in the fridge in the store and you get it home before it thaws. Beneficial bacteria cannot live on a shelf in a bottle as it needs oxygen and food to live. It lives a few weeks in the fridge so only buy the refridgerated stuff or nothing at all.

DO NOT buy test strips. Strips start to break down when you open the container and expose them to air. Buy liquid test kits.

DO NOT buy pH Neutralizer. Goldfish are fine in any water that you can drink. Tap water is often between 6 and 9. That's fine.

DO NOT buy all sorts of medications. Pristine water is all you need to keep your fish healthy. If your fish get sick, test the water and step up changes. Pristine water is so much better than medications and some medications must be used in a hospital tank or bucket because they will kill your beneficial bacteria.

DO buy aquarium gravel for the tank bottom. SOme beneficial bacteria will grow in this.

DO, for goldfish, buy a filter rated for 3-4x the tank size. DO NOT worry about buying extra filter cartridges. You can rinse the cartriges you have in old tank water when they get dirty, but remember that the good bacteria is on there so don't rinse in chlorine water. If you change the filter completely your water will no longer be cycled and your fish could die. They need that bacteria colony.

DO NOT buy any chemical that "adds" slime coat or stress coat to the fish or changes the pH or reduces ammonia. All you should have is chlorine and chloramine neutralizer. Stress coat reduces the dissolved oxygen in the water. Your fish don't need help with their slime coating if you have good water conditions.

DO NOT worry about carbon for the filter. Its only active for a week and then its just another porous surface for the beneficial bacteria to grow. Don't change it often.

SO in the end the bare needs for healthy long-lived goldfish...
Large tank with top
Master test kit
Gravel vac and some buckets for water changes
Filter
Dechlorinator
Gravel
Food
Fish
Water
Research
A good forum to ask questions (here you are)

The initial investment for a ten gallon fish tank in the US is $176 dollars. SInce you have goldfish you will need more space than that so please do your research before getting pet fish.