Glad you asked this, Olympia! I've been researching into this, as well - I love the look of it, and I know my loaches will adore them.
There is n o need to replace them, they can stay in the tank permanently
Won't they eventually rot away or disintegrate eventually? I've read that they will release tannins, and have a similar effect as driftwood (though less) - is this true?
Do they cause any changes, otherwise - for example, will they cause ammonia or raise nitrate levels as they decompose? Or does this not really apply because they're dried leaves.
ANY oak? I have White Oak and Pin Oak in my backyard - would it be safe to just pick them up off the ground when they fall this autumn?
It seems kind of dangerous! What are the chances of molds being carried into the tank, or . . . do you clean them somehow?
Sorry for all of the questions, I've been wondering!