Stressed Yellow Tang

[Lampshade]

Last night i added a hippo tang to my tank, and everything seemed to go well. My yellow tang is usually fairly agressive, and was tail whipping the new fish for awhile, but after a couple hours seemed to leave him be. I was up pretty late and did a water test, and noticed my yellow tang gasping at the surface, but only for a min, then went back to the rocks. So this morning I've come to find my yellow tang hiding behind a rock, on the sandbed. It seems to be panting, but there are no discolorations. All other fish seem to be doing well, the hippo is still hiding, but seems happy. The hippo has taken over the yellow tang's "spot" in the tank. So.. is my fish just stressed? will it go away?

I also changed out a powerhead for a kora 4 so that i stop losing snails in the aquaclear. Coral's aren't too happy about this as they adjust, but now i'm wondering if it's the powerhead, or if its something in the water. Either way, i'm going to do a 25% waterchange once my SW mix is ready. hopefully this will clear some stuff up, and not stress out my yellow tang any further.

Tank tests:
Salinity: 1.023
Ammonia: 0
Nitrite: 0
Nitrate <15
KH: 10 (usually higher, been trying to bring this up the past couple days)
Calcium: 460
PH: 8.3 - 8.4 morning to night.
Temp:78 - this one could be major, since last night the temp was 84, so i left the fan on overnight(usually on a timer with the lights)

I've looked through a lot online, and i've seen that usually yellow tangs usually show the lines down the sides when they're stressed (the ones from nighttime), which my fish does not have. I don't imagine that i have ammonia poisoning, since i'm under a measureable range on my test kit, and also my fish does not have red coloration around the gills, and the rest seems fine.

anyways, if anyone has an idea of what could be up, let me know. thanks in advance.

[Zoaelite]

So.... I've had a 55g set up for about 9 months now

If this is your current tank size then that's your problem, one tang in a 55 is high stress 2 tangs in a 55g is to much. I would remove both of them or upgrade the size of the tank.
Levi

[naesco]

Lampshade, Tangs require 6 foot tanks. Your tank is less than half of what they need.
Remove the blue tang (which gets the size of a pie plate) and return it to the LFS as soon as you can.
Make arrangements to re-home the yellow tang until you upgrade to a tank large enough to accommodate tangs.
There are tons of other fish suitable for a small tank that you can choose from.

[Lampshade]

Tang's up in the FS forum, yellow tang's still small and has been fine so far. Moving in a couple months and doing a large tank upgrade during the move, this should allow room as my yellow tang grows, and then i'll look at getting another. I've seen 2 tangs in a 55 before, but, it was a fish only tank with lots more room than i have. Should have done more research, i usually do, just got carried away at the LFS. Thanks for the replies, and if you want a tang...
http://www.canreef.com/vbulletin/showthread.php?threadid=65685

[naesco]

Tang's up in the FS forum, yellow tang's still small and has been fine so far. Moving in a couple months and doing a large tank upgrade during the move, this should allow room as my yellow tang grows, and then i'll look at getting another. I've seen 2 tangs in a 55 before, but, it was a fish only tank with lots more room than i have. Should have done more research, i usually do, just got carried away at the LFS. Thanks for the replies, and if you want a tang...
http://www.canreef.com/vbulletin/showthread.php?threadid=65685

Good Job Lampshade, It is easy to get carried away at the LFS.
When you upgrade go 6ft and you can choose from many tangs,
Study them here http://wetwebmedia.com/besttgsreefs.htm