Please Help

I don't know if I'm missing something or just have a recent string of bad luck. I had one fish die on me a week ago (a purple dottyback). I had my water tested about 4 days before this and it tested as followed (by LFS):

Saltinity: 1.026
Nitrate: 5
Nitrite: 0
PH: 8.2
Phosphate: .16

Now about a week has gone by and I wake up to a died Flaming Dwarf Angelfish. I tested my water before I conducted my weekly 10% water change and the results are below.

Salinity: 1.026
Nitrate: 5-10 (the color is something in between 5-10)
Nitrite: 0
PH: 8.2
Phosphate: .25
General Hardiness: 180
Carbonate Hardiness: 240

I have a 60 Gallon Tall tank with an open brain, a few red mushrooms, a Blue hippo tang, and a Ocellaris Clownfish. I have about 3 inches of live sand and about 60 lbs of live rock, a power head that has great circulation and on top of that I still have a biowheel electric filtration system rated for a 90 gal tank that has additional carbon media and zeolite crystal media inserts. I do weekly 10% water changes using RO/DI water from my RO/DI filteration system as well as TM reef pro salt.

What I'm I doing wrong. I'm about a week or two out from having my sump/refugium installed, but my water levels seem fine to me.

 

Also, there did not appear to be anytype of fungus, desease, or parasites on him. My blue tang will have ussually one or two white dots of Ich from time to time but nothing else.

 

Have you QT'd all your fish before adding them? If not, it's possibly you've got some sort of internal parasite, whether it be flukes, worms, etc.

In addition, you have placed some potentially large fish (which need a lot of room) in a relatively small tank, of greatest note the Hippo tang. This will cause undue stress on all fish, not just the tang, as it will be harassing other tank mates to some degree due it's own stress.

 

The Tang I understand needs to be moved to a much bigger tank in the future, however it is currently 1.5 inches long. I have plans on moving it to my much bigger tank that is being built. as far as stress, I have not seen any bullying happening, in fact the tang and the clownfish swim together and basically are only a few inches apart at all times. I do QT all the fish I have, but have not added a new fish in quite some time.