Saltwater Aquarium chemical levels

Question
I have a 45 high tank that has been in continuous operation for 15 months. Recently, after a 1/4 tank water change all my livestock died off.  I had a tomato clown, a 3 y/o percula clown (that made it to Hawaii from the mainland in a water bottle) and 3 lizard fish that I captured here in Hawaii.  I tested the water and found that all my chem levels were jacked up.  PH is low at 7.8 - 8.0, Ammonia is high adn 1.0, Nitrite is high at .50 and Nitrate is right at 40.  How do I correct the low PH and Ammonia.  I know that another water change should help with the Nitrite and Nitrate levels, but am hesitant after killing off my fish after the last change 2 weeks ago.

I am currently running a Power filter with Black Diamond carbon & PhosGuard phosphate/silicate control.  I am also running a protein skimmer.  I have an undergravel filter as well with 2 uprisers which I believe are putting plenty of oxygen in the water.  The substrate is gravel and I have one 5 pound live rock and 2 dead corals.

Help!!!  How do I fix my problem?

Answer
hi Cody,

   There are no words to describe the loss one feels when a beautiful creature has beaten the odds and survived, only to lose him to an unfortunate incident. First, I would test your water source. Then I would check to make sure all the filtration and supply lines are free and not clogged with a dead animal. Ammonia is a by product of dying material and definitely should not be present after 15 mos. of operation. Has anything else changed besides doing a water change? You will have a cycling situation now and should probably only use a couple of damsels to aid in recycling. I would also do another major water change and see if readings improve. Check your system thoroughly and get back to me with your findings. Gerry.