Sounds like something is in your water that is reacting with something in the water itself bonding to the air surface. There is many items that are used in the hobby that causes a change in surface tension. Here is a list of some of the more popular causes and should try to restrict the use: Dechlorinator, stresscoat, frag putty, glues, medications, redslime remover, bacterial additives, New filtersocks, cycling aquariums etc. Running fresh activated carbon and small water changes with RODI water and salt that has been mixed for no less that 6 hours before use.