You'll want to do a 50% water change every 2 to 3 months. This is most noticeable when the water begins to have a yellowish tint. When changing the water, you will not remove the frogs, sand, rocks or gravel. Replace the water with aquarium-ready water. In some parts of the US, you can let tap water sit for 48 hours (which is ample time for the chlorine to evaporate).
When changing the water, you will want to disperse the water so it is not poured directly onto the rocks/sand. Pouring the water directly into the tank will stir up the rocks and the sand. If this happens, let it sit. It will take a few days for the sand to settle. If it doesn't completely clear within a few days, do another 50% water change to remove the floating sand. It will typically settle back down to the bottom of the BioSphere.
We suggest using a coffee cup lid (like a Starbucks lid) turned upside down and pour the water into the lid so it disperses the water. You can also use your hand and pour the water over your hand with spread out fingers. An inexpensive, small size watering can is also a fun way for your kids to help change the water as well.
In many areas the chloromine levels are unsafe - and chloromine will not evaporate on its own.
For the safety of these critters, we recommend distilled water (one bottle ought to do it) or using a pet-store water conditioner with your tap water.