I'm a pest controller by trade in the UK, we tend to get them this time year and have various names for them, filter flies, bar flies. It's possible you have had a few lay eggs in your tank and what your seeing is a hatch, as they tend to hatch around the same time this could be why you have got so many. Check the tank for larve around or just below the surface, it's unusual to find a group gathering to lay eggs, far more usual to find them in large number close to where they have hatced.
Are they slow moving small flies that just seem to float around in the air?