As for ammonia in an aquarium, bacteria present will convert the ammonia to nitrite, then to nitrate which is stable under normal conditions.
FWIW-I suggest that you get a good aquarium reference book, as you really should understand something about these processes before you begin being responsible for the care of any creatures. I don't mean to be harsh or critical, but it will save you much heartache, $$$ and frustration (not to mention the lives of the critters). We ALL started out wanting an instantly beautiful aquarium, but that takes time and patience (the most important thing you will ever put into an aqaurium).
My personal favorite (out of print, but available):
http://www.amazon.com/Marine-Aquarium-Reference-Systems-Invertebrates/dp/0939960052">http://www.amazon.com/Marine-Aquarium-Reference-Systems-Invertebrates/dp/0939960052</a>
Prepare to learn, it will be a fascinating journey, and many people on here to help along the way.
JMHO, good luck-