It could be a mini-cycle problem, in which the tank was not cycled correctly when you first started it. Then again, you said you had it for 7 months.

This is what you should do:

Vaccuum the gravel every other day.
Remove 25% of the water.
Add stress coat.

I don't think that it could have been because of the driftwood. However, I have heard that some people put new driftwood in boiling water and then let it bake in the sun for 2-3 weeks, then boil it again. This ensures that all bacteria on the driftwood will die.

Also, what type of fish do you have in the tank? What are your ammonia levels and hardness levels.

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