I lifted the lid which is over the filter today, to see a 'family' of little insects. They're tiny. At first I thought they were fry swimming back and forth just under the surface of the water, but on closer inspection I realised there were a good few lined around the edge of the water on the side of the tank (only within the confines of the filter area (Juwel), and that they were actually dashing back and forth ON the surface of the water. They're so small it's difficult to see them properly - but they look a little like what I'd expect mites to look like...! Any ideas?