Yep, tannins from the wood in the tank. It's more noticeable in smaller tanks like yours, but tannin leeching happens regardless. The tannins lower pH and help to soften water, and can benefit fish that prefer these conditions (like bettas).
Water changes will help get rid of this discolouration if it's not what you're after. However, I do suggest looking into blackwater tanks. They're very interesting, and prove to have quite a few benefits for fishkeepers (and their fish) depending on what they fancy in an aquarium.
Hope this is helpful!