Cause is bad water quality. Test your tap water ammonia and nitrite, then if both are 0 ppm, do at least a 70% water change with warm, dechlorinated water to get nitrite to under 0.25 ppm, 6 hours later, do another 50-70% water change to get it lower and keep doing them until it reads 0 ppm or as close as possible.
The glass cats need a larger tank and a larger group even now, as do the bronze corys (of the albino colour morph) and the upside down catfish.