When cleaning up around your aquarium, be careful with chemicals. Things like spray bottles of glass cleaner may seem harmless, but the chemicals can get into your tank and really messing things up. Windex for example contains ammonia, among many other harmful chemicals, so introducing them into your tank on accident will obviously have a potentially dangerous effect. Instead of chemicals, try using water to clean the outside of the glass. If you must use a glass cleaner, spray it onto a cloth in a completely different room and carefully glean the tank.