How large have the tangs you've attempted been? And were they attempted in your 29gal? Tangs get very large and need a LOT of room to swim. Your 90 will be the LOW end of the limit for this particular tang, as they are very active swimmers and move very fast, especially when frightened. Have they been eating, both when you purchased and after you got them into your tank? Tangs are very prone to ick, and any additional stress will bring on an outbreak. Be sure you provide plenty of places for it to hide, and feed a wide variety of foods, including BOTH carnivorous and herbivorous foods. Many people will provide one, but not the other. A good blend would be Formula 1 & 2, Frozen Brine, Frozen Mysis, Frozen Squid, and Seaweed Selects on a veggie clip. This will help with the nutritional aspect, and will allow the tang to decide what it wishes to eat. I have 2 blue hippos, one juvenile(3") and one adult(11"), and I can say once you have one that is doing well, you'll love it.