Not only are tannins not harmful, they’re actually extremely beneficial. Some die hard advocates would go so far as to say they are an essential missing link in softwater aquariums. Tannins/Humic/fulvic acids may even play a large role in how fish are able to survive in ultra low ph environments. They are natural chelators that help make nutrients available to plants, they help bind heavy metals, and some botanicals like Indian almond leaf have strong anti-bacterial and anti-fungal properties.
You always have to be cautious when adding organics to a closed system, but I swear by the use of Indian almond, alder cone, and other humic extracts. Glosso factory makes a nice premade mix call darkwater as does dennerle called Humin Elixir-both if which do not really stain the water too bad. I use both intermittently along with a homemade brew and try to keep a few Indian almond leaves right in the tank.
Don’t fear the tint!
Experience: worked in fishroom of Eddies Aquarium in Albany all through college, 20 years of keeping reef tanks, planted tanks, and dart frog vivariums.
Current/in-progress tanks: 20 gallon hi tech tank in office, 120 gallon nature/tetra aquarium at home.
Philosophy: work with what you’ve got, and make choices based on the conditions that easiest for you to maintain over time.