well if the water is able to evaporate from the tank so you have to top it up this is not a closed ecosystem now, is it?
in the earths ecosystem nothing is lost and nothing added - but in a tank you are adding minerals with water, food...
if you wanted your tank to be a closed ecosystem than you would need a very large tank with a lot of plants and not a lot of fish. it would also have to be sealed so the water couldn't evaporate (so that it returns unchanged in the form of drops that condense on the cover and lights) and that no gas could be exchanged between the outside and the tank - there should be enough plants to provide oxygen, food...
but than I don't think you would enjoy a tank like that a lot, it would just look pretty bare...
just my 2 cents...