angelfish keep dying

Question
I have been keeping freshwater angelfish for 8 years. I am very knowledgeable about their care. My current tank is 75 gal, and has been set up for ab. 6 mos. After the cycling period, I began to purchase groups of 2-6 juvenile angels, a few weeks apart. Over the months, I have 3 mature adults, healthy and thriving, but all the others have died, fins rotting away, hanging out at the top back corner of the tank. My 3 angels, 3 clown loaches, and 1 pleco are fine. Water is changed once/2 weeks, PH 7.2, temp 80 degrees, ammonia and nitrate zero. Also, I have not witnessed agression. Why do these angels not stand a chance in my tank? I'm starting to feel like a beginner:)

Answer
Check out the place you are purchasing the fish from, things that can affect the juvenile fish you may have overlooked such as when are their fish delivered, buying on shipments days could increase your mortality rate, find out if their fish tanks have individual filters or all on one filter system-any other tanks have any illness and are still filtering eventually effects their whole system which could be causing your problem. Being you have so much experiance with Angelfish, I'm trying look at things outside of your tank that could be affecting the fish. Your water conditions are in great shape so I can only wonder if you are getting them shortly after delivery and they are so stressed they are becoming ill and can't bounce back or if there is an issue with the tanks where you are purchasing the fish. Do you add some kind of stress reliever for the fish when adding them to your tank? Do you add the retailers water to yours when adding the fish? Do you aclamate the fish slowly? Have you monitored your angels at night? I ask this because I once had one angelfish who seemed okay with the other angels during the day kill the rest of the angels on night, that was a mess to wake up to. This is difficult to monitor but may let you see something you otherwise would not during the day. Best of luck...