My mollie who is the only fish in the tank so far seems to spend most of her time on the surface gulping in the air. I'm pretty certain I haven't overfed her, what else could be the problem? Or alternatively, is this common behaviour for mollies?
Could be a gasp for air from a couple things. The first and easiest to fix is a simple lack of dissolved oxygen in the tank. Add an airstone and see if that helps. The second could be the results of either parasites or damaged gills in which case the cause needs to be identified immediately. Check your nitrate, nitrite and ammonia levels. Try to look at the fish's gills and see if they are red or swollen, if they are, post in the disease section to get help.
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