OohFeeshy
It's only forever; not long at all...
I've been wondering about this for a while, don't laugh
But what actually is stopping you from using ordinary tea bags to create tanninated water (ie, for bettas). Its still tannic acid, right? Surely if its suitable to drink, it's fish safe? Or should I keep my teabags for me? Answers on a stamped addresses post below, s'il vous plait.

. I'd use oak leaves if we had any oak trees, and all the leaves are swept up very quickly anyway. I remember what I was going to post though. The only problem is 'tea scum', I have hard, alkaline water so the tannic acid reacts with it, causing a nasty scum on the water. I was planning on experimenting with ion exchange resin anyway (proably buy some cheap water filters), that should clear it up. And also help, since the majority of fish are from softwater area anyway.
so suggest you do some further research.