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
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.
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.
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you can buy it from the store, it will require level 32 to unlock.sources :http://www.yayeasy.com/2009/10/happy-aquarium-crawler-coralsdecorations/
It will cool down water temperature when by melting down AND it will lift up the water level in the aquarium
Evaporation.
water level in aquarium not high enough or if it is, add some water to the filter equipment and sound will stop : )