It sounds like you may be looking at several problems. First, as has already been mentioned, that starfish my endanger your corals. It would be best if you gave it back to your fish store if you can. Also, it sounds like you have stocked too many fish too quickly. I have found the best success by adding only a few fish every month or so. Give your tank time to stabilize and then add more as long as your ammonia and nitrite levels don't start to rise.
Basically your tank has a biological filtration system. This is composed of a group of bacteria which live in your filters and sand. This bacteria consumes fish waste and helps keep your tank clean. (If you already knew this, sorry... just restating). When you add fish, you increase the bioload on your system. It takes the bacteria about a month to grow up to the new level needed to keep up with your fish waste. That is why if you add too many fish too quickly, you will not have enough bacteria in your system to handle all the waste. When this happens, toxins in the water will start to rise and you may face a tank crash, killing your fish.
Your pH sounds too low. It should be around 8.3. If you haven't done a water change in a while then I would start here. Changing your water is one of the best things you can do to get everything back in line. If you need to raise the pH you can use baking soda or pH up solution... but do it slowly. You need to give your tank a few days to adjust to any changes you make.
When it comes to dosing your tank with chemicals... first you need some test kits. Never dose anything you don't test for. What chemicals you need are dependent on what your particular tank is consuming. Start by testing for calcium and magnesium. Once those levels are known, then you can start dosing for that. You have some corals and so your calcium levels will depleat over time. You will eventually need to dose your tank with calcium... but first test your water. Regular water tests will show you how much calcium is being consumed on a regular basis and you can use that to know how much calcium you need to dose. You need a magnesium test kit because calcium and magnesium form a careful balance in the tank. If one goes too high, it will cause the other to crash. You don't want to add too much calcium or else you will lose your magnesium and your corals will suffer.
I would start by testing for calcium, magnesium, iodine, strontium and alkalinity. If any of these levels start to drop, then that will tell you what you need to start dosing in your tank.
