The tanning of the water is actually a somewhat good thing. It can soften the water and Discus like soft water. You can however just soak the wood in a bucket for a few days and eventually it'll stop changing the waters color. 12 will be too many in a 120G, Ever see how big Discus get? Spread your hand out, they will get bigger than that. The more water changes you do, they faster they will grow too.
Remember, you don't have 120G of space, you gotta subtract space with the bottom gravel, decorations, and plants. I'd say 6 or maybe 7 would be the max. But hey, do what you want. I've seen that many in a 75G done successfully. He just changed the water every other day!
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