Don't add any uncycled live rock to your tank, either.. you'll end up killing what you already have.

They keys to keeping your 12 gallon happy are waterchages, waterchanges, waterchages... waterchanges export toxins and nutrients, which is especially important when you don't have a skimmer. They also replenish calcium and other elements used up by your critters. I do weekly 20% water changes on my 12gallon. When you are having problems (like now) I would run carbon and something like purigen, but not on a regular basis. Also, I wouldn't bother dosing... if you overdose, you can cause more problems... just keep up with your replenishing waterchanges. If you have a nanocube, get rid of the ceramic rings and only keep sponges in the first chamber. Make sure that you clean these each time you do a waterchange (don't use soap! just rinse them in RO or salt water and get all of the gunk out). If you have an eclipse 12, then don't run the filter media all of the time... just put a new one in once every week (or couple of weeks depending on how much particulate matter you have in the tank) and run it for a few hours. If you leave it in there, it just becomes a nitrate factory. When you do waterchanges, use some airline tubing and suck detritus off of the rock and sand. You don't want gunk sitting around in there. Keep up with topoffs (make sure you are topping off with fresh water) so you don't have salinity swings. Your temp sounds like its fine, just make sure that its not fluctuating. Make sure that you have enough light for the corals that you are keeping. If you are keeping high-light corals or anemones under stock nano-cube lighting, then they are goign to die and crash your tank. Keep your bioload low. Three fish create more toxins one. The more fish you have, the more likely you are to have problems. I just have a single damsel in mine. Lastly, make sure that you do NOT overfeed. I just feed my fish and shrimp two to three small pellets each. You don't want any uneaten food in the tank. A small system just can't handle it.

Hope that helps...