Empty Aquarium

I bought a medium size of aquarium in the house.It is my plan to take care fish in the house as part of relaxation.When I arrived from work it is so relaxing to see the aquarium with different kinds of fish.I enjoy watching the fish swimming and diving to get food.After, three days I notice that aquarium look blurred.The water was already dirty.I told cousin to clean the aquarium.We are not both aware the proper cleaning of an aquarium that half of the water was set aside.It will be added to the new replenish one so that fish temperature will not be disturb.It is too late before I know this.Two days after cleaning I notice that fishes peck food.Three days I feel so sad to see that all my fish sink and gone :-(
Maybe, I gonna read first clippings how to take care fish in the aquarium before I buy a new one.