Alder cones can make water too black :huh: .. If too much, no fish can be visible.. So due to this I use it only in the tank, and just a few, if black water is more acceptable for some shy species I can use more, but usually they are strong enough to use only some pieces. Often I have to remove them after week or two, because water start to get the same colour as strong black tea.. However fish do not seem to have any trouble to live their normal life at very dark water, f.e. to catch food, lie eggs.