A few things...for a Freshwater tank

The first day, you are about to come home with a fish tank, you might have a picture of a colorful tank with fish swimming all around, eating food and listening to what you say. I wish I had known a few things before:

• Beginner's should ideally start with a 20+gallon tank. A smaller one works, only if you want to have 3 or 4 of them. 
• Thumb rule- size of full grown fish x number of fish = gallons required. 
• The filter size, should be on the higher side- for 20 gallon, you might go with the filter for 20-30gallon tank. This will help to balance ammonia and nitrite level in the tank. 
• For fish that lay eggs, like Tetras, be careful when you gravel vac the tank. You might scoop out some of the eggs and it will certainly cause stress, aggressive behavior. 
• Overfeeding is a common thing. Feed as much the fish eats in 3-5mins. 
• When you move decor around or introduce new fish, or clean make sure the aquarium light has been turned off. The heater as well, during cleaning and water change.