Just curious: Does the water in fish tanks evaporate?

The water in fish tanks does evaporate. Having a tight lid with no gaps slows this, but doesn't stop it. However, it's just the water that evaporates - the dissolved nitrogenous wastes, minerals, etc. stay in the aquarium and don't evaporate out. So as aquarium H20 evaporates, the level of wastes becomes more concentrated in the water that remains. So as the water naturally evaporates, the water quality gets worse and worse, and topping off with new water to make up the for the evaporation over time doesn't make up for that. That's why it is important to do weekly water changes where you remove 25-50% of the aquarium water and replace it with fresh clean water that has been dechlorinated and is the same temperature as the old water, to remove the wastes left over from evaporation that have been produced by the fish at the same time as you add more water to make up for the water loss from evaporation.

Yes water evaporates eventually no matter what kind

It Could be since Freshwater fish do not actively drink water, but absorb the water through their skin and gills. On the other hand, saltwater fish do actively drink sea water. Their gills process the water and take out the salt.

yes

yes

yes