well its probably a good idea to fully clean the tank at least once a month because the frog is going to deficate and some of the food is going to die. the waste and dead food can lead to disease which can severely hurt the frog. you're probably better off having a bowl of water in there instead of the water covering the bottom of the tank, so you can just empty the bowl daily as opposed to emtying out the tank.
so to wrap things up.
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a more naturalistic approach.
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cover the bottom of the tank with about .5inches of non-fertilized soil.
plant some live plants in the tank if you like (they provide good shade for the frog). petstores i.e. petco sell live plants for a few dollars each.
then place a bowl on top of the soil for water.
cover the soil completly with the moss.
place some logs/smooth sticks at varying levels for the frog to climb on. (they are aeboral, meaning they like to climb up.)
this setup is pretty easy to maintain and the frog will be realativly happy. you can use a 40w bulb from the hardware store to add some heat/provide light for the plants, but make sure it stays humid enough in the tank so the frog is comfortable.
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or
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you could cover the tank bottom with "reptile carpet" and place the water bowl on top of it and add some sticks and logs for the frog to climb on.
this setup is a bit easier to maintain, but the frog may not be as happy.