Notator said:
I suppose in nature allsorts of things drop into the water/rot/leach etc - but on the other hand there is "new" water on the way down all the time...
I was just worried that in the confines of my tank it wouldn't be "flushed" in the same way - but point taken!
Yeah , no kidding. Competition tanks have nothing to do with nature or with fish. Our tanks are ornaments and are for our viewing pleasure. In fact no tanks have anything to do with nature.
So I think you might be missing the big picture. "New" water that flows into these rivers and streams comes from rain, and when it rains on the water it rains on the land. The rain on the land absorbs the tannins from fallen leaves, bark and sediment, and this effluent then flows into the bodies of water. So the "New" water has a higher concentration of tannins, not less. It seems like you are equating the tannins with pollution, and this is incorrect. The pollution happens when WE dump things like fertilizers and detergent and petroleum products into the bodies of water. The tannins are a natural part of the Amazonian estuarine systems and they are important to the health of the systems.
Just like tea, tannins are a mixture of antioxidants, natural "phenols" and other components which together produce a type of prophylactic. Many of the trees use these compounds to protect themselves from leaf eating animals and to protect themselves from infection. The waters surrounding these trees evolve to use these tannins for all sorts of purposes - and they clean the bodies of water as well as keeping the pathogen population (which multiply quickly in tropical temperatures) under control. So the idea about "flushing" them away somehow is misguided. The more it rains and the more "New" water enters the estuaries, the more tea stained they get and the healthier they get.
For us, the tannins are an inconvenience, you certainly don't want to drink it and it's annoying to look at, but that's about it. In any case, you should be doing large and frequent water changes in your tank, because there are a lot more dangerous things that you yourself put in the tank than tannin stains from Mopani wood. That food you put in every day does more damage than tannins, that's for sure.
Cheers,