Decide what kind and how many fish you want
Research before putting different species together. While some fish are compatible, others aren't. One might speculate that fish would enjoy some activity in their lives, so don't get just one. Angelfish do not go along with Malawian Cichlids. But Corydoras go along with Angelfish.
Also, the general rule is that for every gallon of water, you should have no more than one-inch of slim-bodied fish. When a tank is filled with too many fish, there is excessive waste, food particles, and chemicals floating around, making maintenance difficult as well as the tank unhealthy.
Make sure you can provide any specialized care the fish might need
For example, different fishes need different foods; and some fish require frequent maintenance than others. Some fish are perfectly happy with flakes and can be fed with an automatic feeder, which makes it possible to leave the tank unattended for a week or two.
Look for an appropriately sized tank
For goldfish, buy a tank with 20 gallons for the first goldfish, and 10 gallons for each additional goldfish. For freshwater fish, the one gallon per inch of adult fish theory won't work. There's no way why you would keep a 50 inch fish in a 50 gallon tank. Furthermore, a bigger tank is always better. Even if the fish looks small, it will thrive better in a bigger tank.
Make sure you have all the necessary equipments required for aquarium care
Here's what you'll need: algae scraper/pad, razor blade (plastic blade if you have an acrylic tank), bleach, water siphon, bucket, lime remover or glass cleaner that are specifically made for aquariums, filter media, filter brush, old bath towels, and paper towels.
Feeding
The most common aquarium fish should be fed two or three times a day, but each feeding should consist of only as much as is consumed in two or three minutes. This takes a little practice, and the careful aquarist will actually time his feedings occasionally, to be certain that the proper amount of food is given. Fish could probably be fed 10 times a day without problem, but one overly generous portion every two days could cause problems. Uneaten food contributes to poor water quality, which causes water cloudiness, rapid algae growth, and often leads to fish diseases. Alternating feedings among flake, frozen, freeze-dried, and pellet foods will provide a well balanced diet for fishes of various feeding habits.
Perform partial water changes weekly
20-30% is a good amount. To do a water change, get a gravel vacuum and siphon out any waste in the substrate. This will pull out water at the same time. Replace the water with fresh water, but remember to treat it first. While doing this, you may place a stiff plastic divider between the area you are cleaning and the fish so that you do not accidentally suck any up in the tubing. Additionally, with the siphon running, you can tap the tip down into the gravel to help remove any fallen debris.
Temperature and other equipment control
Check the water temperature, keeping in mind temperatures will fluctuate throughout the day. Therefore, test the water every morning and then again in the evening. To
maintain a healthy tank, you should keep a chart of your findings so you can identify any problem before they spiral out of control.
Also see if everything in your aquarium is in working condition. Do not let algae accumulate in the tank. Clean it with an algae magnet or scraper on the insides of the glass as well as on the decorations.
Monitor your fish frequently
See if they are doing well in the aquarium. Check for anything strange, such as changing color, damaged tails and fins. Also, it's important that the fish in your tank are compatible with one another. To be honest, cleaning your fish tank may not be much fun but it's a necessity. For your fish, your fish tank is home and your pets deserve to live in an environment that is clean and healthy.
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