Is all-purpose household ammonia safe to cycle an aquarium with?

Yes, but BE CAREFUL when you get it--it must be "unscented" ammonia, which is hard to find some places. Some people are saying, "huh?" The questioner is asking about the "nitrogen cycle" of an aquarium. Fish pee has ammonia in it, like all pee does. (Yes, fish pee.) Bacteria in the tank will turn the ammonia into nitrite, which is less harmful to the fish than ammonia but still bad, and other ammonia turn nitrite to nitrate which is harmless. (And then, if you have a lot of plants, the nitrate will fertilize them.) There are better instructions on the internet, but the basics follow: You need to get an ammonia indicating card, some bottles of Cycle, which is a concentrated source of nitrifying bacteria, some unscented Parson's Ammonia, and an eyedropper. If you can't get any Cycle, go to a really good fish store and buy a bag of their water, which has bacteria in it. Pour the Cycle, or the fish store water, in the tank and add just enough ammonia to turn the indicator in the middle of the card dark. Then let the filters run. Keep doing this until the indicator shows "no ammonia" after eight hours. When it does that, you've got a nice colony of bacteria built up. At this point, change 90 percent of the water and add a couple of the least expensive fish you're going to keep.