ammonia

this is a take your time hobby. read up on avery thing your going to do before you do it. now when your tank has been up for lets say a year or two and you have a spike like this its probley becouse somthing has died off and then you want to do water changes. there is so much to learn marine bio 101 + when you get into corals keep on reefing and take your time, remerber to read and you wont go wrong