What am I doing wrong???

I have fish that I have had for 4 years yet there are some that I cant seem to keep. Example: I bought a blue hippo from Aquatics around one month ago. Doing fine.... I really like this fish so I went to the Aquarium and got two more $ 40.00 each. They disappeared about 5 days later. I have 4 anemone and an eel so I thought well The one is fine maybe they were sick. I then saw some that looked very health at Bird World last weekend so I picked up 2 more. Well one is gone and the other got thin and died last night. Why do I have one fone & well and now 4 fish and 160 later..... What is up???  I thought this was a fairly easy fish to keep. Stressey but they are fed garlic and should be fine...