Congratulations on your first new fish tank! The carbon media part of the filter should be as submerged in the water as possible, as the carbon needs to make contact with the water to clean the impurities and trap the waste solids.
How big is the tank and what are the sizes of the current fish?
A goldfish should have a 25+ gallon tank and another 15+ gallon for each extra one, as goldfish are very messy poopers and you may have to do a water change soon in a couple of days.
Also, goldfish, though hardy, are not good starter fish for new tanks, as the tank must go through a process called cycling:
When there is fish waste and uneaten food, it becomes ammonia. Slowly over time, good bacteria will convert the ammonia into nitrite. Then, anotehr type of bacteria will convert the nitrite in nitrate, with an A. Nitrates are less harmful to the fish (and friends) but over 25 ppm levels are not good. This can be lowered by partial water changes. Usually, the cycle for a new tank with one or two fish will be about a 4~8 weeks. I know it seems like a long time, but it's for the future of your fish for them to live a long and happy life.
To see how much ammonia, nitrites, and nitrates you have, you can bring a sample of water to your LFS (local fish store) and they will test it for you at a fee (sometimes it's free). OR you can buy a water testing kit, which may be more convenient for you.
After a month or so, until ammonia and nitrite levels are 0, and nitrate levels are under 20ppm, you can start to buy new fish, around 1~3 new ones every week. Please don't overstock your tank. It can create crowding, unwanted fin nipping, and a bacterial unbalance in your filter. Do not go by the myth of one inch of fish per gallon. Consider the adult size of the fish (in this case godlfish are 8+ INCHES LONG) you want to buy and how much swimming space it may need. ALSO fish DO NOT grow to the size of their tank and STOP growing, it is a MYTH.
I know this is probably more info than you expected, but it's all the basics of fishkeeping in one LONG answer so you have more knowledge about keeping fish for the first time.
