Tank dimensions are adding up to 98L/26G tank and it sounds like you've got pretty good maintenance habits there. So nothing is jumping out to explain the "brown water" quite yet... er, you say brown in the title but "cloudy" in the post. Is it a milky white cloudy? That could mean a "bacterial bloom"... or "yellow brown" water could mean you have wood that is giving off tannins. Do you have any wood in there?