New Fishy Friend

If you have any drift wood in the tank, it will more than likely be the driftwood that is leaching tannins into the water.

The rocks could also be leeching something into the water. Take a rock out, let it dry overnight, and then pour some vinegar over it. If the vinager fizzles or bubblea at all, then it isn't suitable for the tank. If it doesn't fizzle, then rinse the rock and put back in.

If it is neither of the options above, then I'm afraid that i's the compost mixture you have under the gravel..

The tannins from the driftwood is usually alright for the fish, just makes the water look "unpleasant" for the viewer.
If it's the rocks, change them.
If it's the compost. Get it out. It could be dangerous for the fish!