06-03-2010, 12:45 PM
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I may not be correctly understanding what TexasTanker posted, but I have never read that tannins from wood will harm the fish. Most aquarists see this as troublesome only because of the brownish-yellowish tint the water takes on. But forest fish frequently live in blackwater streams and these are the colour of very strong tea only due to tannins from decaying vegetation and wood.
It may be that your thought about harm is from the bacteria issue I mentioned previously. I will explain it a bit more now. I placed a rather small piece of mopani wood in my 90g, after I had rinsed it off (it was dry), expecting some tannins but prepared to live with that. A day later this white "fungus" growth appeared all over the wood, and the aquarium water turned not brown but whitish, similar to a bacterial bloom. That was bad enough, but I noticed that all of the corys in this tank began having problems; they became lethargic, respirating very fast--a sure sign of water problems. I pulled out the wood, did a 75% water change two days in a row, and saved the fish, although it took a couple of weeks before the water was again as clear as it had been the day before the wood went in.
I just happened to be in a local fish store that weekend, and overheard a customer asking the staff about much the same reaction from a piece of mopani wood, only he had his corys die because he did not take action soon enough.
The point here is that the bacteria in the wood leeches out when it is first placed under water, so with this wood I would suggest always soaking it well (or boiling) before placing it in an aquarium with fish.