03-21-2012, 05:38 PM
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Yes, more water evaporates with no cover. Water is warm, and warm water evaporates faster than cold, plus it depends upon your room temperature.
With a cover, scarcely any water actually evaporates; it condenses on the cover overnight, then most of it drops back into the tank when the lights are on. Some will evaporate, but minimally by comparison.
Issues with open-top tanks. Fish will jump out; many fish given the chance will jump, especially during night when they may be startled by other fish or something. Dirt and objects can easily get in the tank; other pets like cats, and small children, are issues. Water evaporation being faster will get into the walls and ceiling, much as it does in the bathroom; this has to be dealt with to preserve the structure. Depending upon how close to the water the overhead light fixture is, there may need to be a glass separation to prevent splashes and water rusting the fixture.