To kill the plants and fish with.
My wife always asked the same question.
Some stupid thing I read somewhere (probably dumb Barron's - Plants for Your Aquarium Book) about using roughly 3 inches of substrate. The Seachem Onyx I have finally come to appreciate what it does for the KH of my water, with my tap water having good GH, but the gravel pieces are much larger than they should be. The batch I got. Oh well.
Also, I came back from a 7 day vacation with my engineer buddy feeding the fish. The guy warned me that he has killed a cactus. Although he has a great understanding of organic chemistry - I just need to get him an aquarium. Well, I told him feeding fish food about the size of a dime would be enough. Whoops! Well, the Mollies are fine. But my beautiful 4 Neons are missing.
I am very tempted to put the Mollies at the bottom of all the substrate to be fertilizer for my plants. I won't. But I am tempted. Plants don't eat plants. But Fish will and sometimes do eat fish. Stupid Fish.
Right now I like plants much better than fish.
If going to the LFS was not such a massive pain in the ass, I would trade in my huge Mollies for some tiny Neons or maybe some Raspboras.
Right now getting to the Local Fish Store with my 5 gallon bucket filled with my harvested Rotala Rotundifolia Green to exchange for some Seachem Excel is going to be a pain. Need to do it though. I am just in a grumpy mood tonight.
I now have my 10 gallon tank in front of the window. You should see the eclipse of light in the tank now in the daytime! I hope that Diana Walstad idea pays off!