Adding too many fish to a new tank will often cause cloudy water and possible losses. Start with just a few fish for the first couple of weeks until the biological system is established.
This will cause cloudy water and increase the chance of ammonia pollution that can kill fish. Best to feed what they can eat in a minute, several times a day. Feed something different each time, this helps to give them a healthy balanced diet.
Wrong types & mix of fish
Only keep fish together that need the same water requirements, such as pH and temperature. Make sure the fish are compatible. General rules: big fish eat little fish, and some small fish can get large quickly! Do some research before buying.
Forgetting water changes
Water changes of 25% to 50% every 7 -10 days is the most important thing to do to keep water clean and fish healthy. Just because water is clear doesn’t mean it’s clean, do your changes!
Over medicating / treating water without cause!
Don’t add anything until you identify the problem. A simple water change often resolves the problem.
Not getting the right information
Find a reliable & knowledgeable source for information. Join a local aquarium society if one is in your area.