Water Evaporating In Fish Tank. That’s the reason when you pour water into your fish tank, you will see gradual degradation in the water level in some days. Water is chemically prone to evaporation from heat.

Since pollutants also stay behind when water evaporates, it seems sensible to freshen things up for your betta. Heat is the main cause of water evaporation, so the less heat, the better. Pure distilled water is an excellent option to ensure you are not losing water due to evaporation.