The compound is juglone. The wood is not very high in it, but it is there. Some plants (like tomatoes) are killed instantly by it, and some plants (like most ferns and hostas) could care less. The leaves and the roots have the highest levels of it. Juglone is broken down in a matter of months by active microbes. I wrote an article about it for a local garden magazine years ago, since my lot had lots of black walnuts.
I've never tried it in a planted aquarium. I've used it in a vivarium and have had mosses grow on it and broms anchor their roots on it without problems.
Because everyone is cautious, everyone will tell you not to use it. But I would guess that you would be fine using it, if you soaked it for a few weeks first, changing the water every few days. That would leach out the stuff that would readily leach out. Consider using carbon in your filter when you start out with it, as well.
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