Has anyone had any experience using them?
any down side?
Where can they be purchased?
I use them. They are basically working the same way as Almond leaves in that the release humic acids and tannis that will somewhat soften the water and lower the pH. They are also said to be anti-fungal. I tend to use about 3 cones per 10 gal, or more, depends on my mood. They are not quite as intense as Almond leaves. You can probably purchase them somewhere, but you can also just go out and collect them yourself. Alder trees grow often next to rivers, I have seen plenty along the Rideau River for example. Collecting time is coming now, September/October. It's best to get them before heavy rains set in as that can wash out the good stuff. Are these downsides, not really, they will take quite some time to decompose and give the tank a natural appearance. I use a pump to do my water changes and pay attention that I don't suck them up as they tend to get stuck in the hose and getting them out it a pain.
Hope this helps! :)