If your aquarium is getting lower in water, it means the water is being vaporized by the light and no need to worry, just refill it.
condensation level
The Condensation Level.
Condensation Level
The ideal water parameters for keeping mollies healthy in an aquarium are a temperature between 75-82F, pH level between 7.5-8.5, and water hardness of 20-30 dGH. It is also important to maintain good water quality with regular water changes and proper filtration.
level 10 for and level 20 to unlock, i am using this site for all the happy aquarium infohttp://www.yayeasy.com/game/happy_aquarium/
There is only one way to get the aquarium in Wizard 101. You will have to pass the level.
The lifting condensation level is calculated by finding the altitude at which an air parcel reaches its dew point temperature and condensation begins. This level is determined by lifting the parcel of air adiabatically until it reaches saturation.
If your aquarium is not level it could stress the glass and eventually cause a crack or break; a difference of 1/4 inch is the highest it can safely be
evapouration
what level are you? maybe you may refer to this list, http://www.yayeasy.com/happy_aquarium/happy-aquarium-levelexp-information/
There is a list of how many points are need for each level up to 46 on Gamezebo's Happy Aquarium strategy guide: http://www.gamezebo.com/facebook-games/happy-aquarium/walkthrough
It could be any number of aquarium diseases. take a sample of water and the next fish that dies to your local aquarium store and have them looked at.