Ok here is my rant. PETCO DOESN'T KNOW ANYTHING. If it was up to me, the fish comunity would demand that Petco stop selling saltwater fish all together. They have staff working with fish who don't even own tanks! A Sailfin tang needs a 180 gal tank when it is fully grown. Your 55 was too small for him (not saying that is what killed him but he would have outgrown your tank fairly quickly). Petco's staff is TERRIBLE about knowing what each of their fish need. Try to find a LFS that is locally owned. Alpine Koi in Fort Collins is my personal favorite. Every single person that is selling sw fish in that store has an established reef tank. They know what they are talking about and when they don't, they do the research to make sure they aren't selling you something you can't handle. Also, as for the "water tests" that Petco does, they are using the dip strips. Dip strips can be nice for monitoring your aquarium for large swings but they are extremely inaccurate when compared to chemistry kits. My advice, get a the basic test kit (they are about $30-45) because it will keep your fish healthy and alive and test once a week. After the test kit, find a good LFS and make sure the person you are talking to has a sw fish tank. And my last piece of advice is simple. DO YOUR RESEARCH. Get on the forums and say "Hey guys this is what I have and this is what I want to get." The people on here will help out and then go to the internet and look up the fish you are getting. Make sure you aren't putting a fish into a tank that it will grow out of. Fish stores put fish in small tanks because a Naso tang, when it's small, only needs like 40 gals to swim around in but he won't be small for long! They can do that because they are going to sell him. Ok so there is my rant. I HATE Petco and refuse to ever buy anything from them again when it comes to fish.
Casey