When setting up a new Aquarium these are some common mistakes beginners make
Setting up the aquarium
When setting up an aquarium make sure you have adequate filtration, most filters will say how big of an aquarium they are for, but keep in mind you may require more filtration if you are keeping large messy species (ex. Large Cichlids, Goldfish). Also keep in mind that you should declorinate your water first with a water declorinator(make sure it will remove both chlorine and Chloramines(these chemicals are added to make the water safe for human consumption by removing harmful bacteria, but will kill fish). Make sure that your tank is running for about 2 days before you add any fish. This helps put oxygen in the water
Water changes
Only remove about 20% of the water out of your aquarium at a time, this can be done weekly or every two weeks depending on the types of fish you are keeping, larger messier fish such as large cichlids or goldfish will most likely require weekly water changes. When changing water use a Gravel Vac, I like the ones that have self-suction, they siphon uneaten food and fish waste from your gravel bed that will pollute your water.
Fish Compatibility
When selecting fish for your aquarium make sure you do some research on the species you would like to keep. Many beginners randomly just put whatever fish they like together which is a big mistake. Some species can grow very large(ex. Oscars, Bala Sharks, Common Plecos), some are predators (ex. Cichlids), some fish are social do best in schools of there own kind( ex. Cory cats, Tetras, Danios, Livebearers, Barbs), and some species can be aggressive to there own kind(ex. Bettas(males only), Red-Tailed Sharks). Goldfish are cold water fish and should not be mixed with tropical species as they have different feeding and temperature requirements.
Tank Size
Another thing to keep in mind is tank size. When selecting fish choose fish that will be appropriate for the size of your tank. Don't Buy a 2 inch baby oscar and expect it to live it's entire life in a 10 gallon it wont happen. Oscars require a minimum of 55 gallons with good filtration. As a general rule you should have one gallon of water per inch of fish, the exception to this rule would be with large cichlids and goldfish as they are messy eaters and produce large amounts of waste. Goldfish require a minimum of 10 gallons per fish, but I wouldn't advise keeping a goldfish in a 10 gallon, its way to small. So keep in mind that your fish will not stay the size it is now(it's a myth, if they do stay small they are stunted and sickly and won't live long) it's best to do your homework on species you would like to keep first before buying them.
Adding your first fish
When adding your first fish to a newly established tank it is best to only add one or two fish in the beginning, because there is helpful bacteria in the water that helps eat fish waste and a newly established aquariums do not have this bacteria yet because they require fish to survive. After you add your first fish make sure they are doing well feed them once a day small amounts, after about 3 weeks you can start adding a few more fish to your tank gradually, as time goes on your bioload(amount of helpful bacteria) with increase. I recommend Danios they are a great starter fish and are very hardy.
Overstocking
Nothing can be worse than an overstocked aquarium, as a rule you should should have one gallon of water for every inch of fish, however there are some exceptions to this rule.
Overfeeding
Overfeeding is a major problem you should not feed your fish more than they can consume within a few min. Uneaten food just pollutes your tank and can create more ammonia and nitrates which are not good for your water quality.
