You'll want to do a 50% water change every 2 to 3 months.  This is mostly 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 chlorine to evaporate). But in many areas the chloromine levels are unsafe - and chloromine will not evaporate on its own.  For safety of these critters, we recommend distilled water (one bottle ought to do it) or using pet-store water conditioner with your tap water.