Fishtanks without a cover is not so good

Although fishtanks without a top are good for viewing fish from the top, it gets just as easily to collect a lot of dust falling into the tank to make the water dirtier as well as tank water evaporates a lot quicker, especially if there is a heater installed. For my big fish tank, I lose close to about a litre a day. The water in this tank is the worse out of all of my other tanks. I am planning to upgrade this tank in the near future.