Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Usually it is not trace fertilizers. I know one product is dark brown in the bottle, but you use so little it does not affect the water color.
I am sure in your case it is the compost.
Ditto the above suggestions:
The material will keep leaching tannins for quite a while, but the more often you can change the water the more tannins you are getting rid of.
Purigen will remove them, and activated carbon. You will have to weigh the costs of these materials and decide which combination works best.
Water is almost free, but you need to do a lot of work to make use of it.
Purigen can be a bit expensive, but can be treated (it is cheap to treat) and reused for several uses.
Activated carbon cannot be reused, but is cheaper than Purigen just looking at the initial purchase price. When you compare the reuseability of Purigen, though, the AC might be more expensive.
You can sure do all three, in whatever ratio you want.