Alder cones are a great source of Tannins and Humins, much like Catappa or Indian Almond leaves. These Tannins are a great source of natural antibacterial and antifungal, which means any product containing them helps protect your aquarium buddies from sickness and help them recover from injuries. As the name suggests these can also stain your water a yellow/browny colour depending on how many you add in. Humins are beneficial for plant growth and lower your pH.
Both help imitate the natural habitats of fish that naturally occur in blackwater rivers, which stimulates spawning and reduces stress. Alder cones happen to be particularly rich in Tannins and Humins. Even a small amount of alder cones can give the aquarium water a nice natural stained look and offer all the benefits discussed above.
We recommend 2 Alder Cones per 20 litres of water, ensure you replace them monthly, if you don't think they are having the desired effect then feel free to work your way up from there. It is also advised to wash the cones before adding them to your tank as while these have been cleaned, you can never be too careful!