Hi Christian,
Your english is fine. It's probably better than some people who use it as their primary language.
I don't have a lot of experience with frog terrariums but some of my friends do. I go with them to frog meetings sometimes and learn what I can.
It is my understanding that water will be brown for one of two reasons. First, if the roots of a plant are not cleaned properly they will introduce soil into the tank, thereby causing it to become brown.
Another reason is if dead leaves from plants are leaching tannins into the water.
So unless your brown water is from frog feces or dead insects, I'm not sure a plant will do much to filter the water.
But to make sure, perhaps you should post this on the Poison Arrow Frog forum on kingsnake. A lot of people post about frog vivaria and Mantellas over there so your questions will not be out of place.
Sorry I could not help more.
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I have 3 mentella viridis and false bottom set up. My water always become brown, I change the water 1 time a week... Can I have help...
Do you know a plant to help filtring my water... I have the mecanics but I would like to use naturel plant to do a better job and to put some naturel beauty...