Why do tadpoles always swim to the top of water?

Some fish, like beta's, will need air. They have to get oxygen every once in a while. For lack of oxygen in the water, you might want to consider activated carbon rock pouches in to your filtration system, or connect an air pump and air stone in your tank. air pumps and stones do not oxygenate water, that is an old myth. a bubble moving to the surface only causes water movement, in order to get more oxygen in water you must either have live plants or some sort of waterfall that will break the surface tension dragging air in and allowing expelled gasses to be released at the same time, carbon is a good idea it will grab the excess nutrients that are using the existing oxygen. air pumps and stones do oxygenate the water by moving the surface of the water allowing gas exchange.

The circulation of water from bottom to top in the tank is what causes most of the oxygenation.

Another reason may be water temperature, If you have a large tank, and the bottom of the tank's water is colder then the top, your fish may require a level of heating in the water. I.E. Tropical Fish Tank.

its better to clean the water and add a lil plant from pets at home. or it can be coz they need oxygen or because they're trying to escape from something in the tank, aka ammonia or a different desiese. i could be because they are eating small particles of dust at the top.

Read more: Why_are_all_your_fish_swimming_at_the_top_of_the_tank_if_the_ammonia_and_nitrate_levels_are_ok