sick damsel need help!!!!! :'(

[CARM]

My yellowtail damsel is just sitting in the sand underneath an overhang of LR and is breathing very rapidly, sometimes even laying on its side. This is very sudden and VERY unlike him, who usually darts around like he runs the place

I have to move the 30 gallon tank he's in downstairs in a few days, so yesterday I transfered the damsel as well as 2 clownfish to an extra 10gallon (small I know, but it's only temporary, and the fish are still small about 1") until the dt is ready; He seemed fine yesterday but today I came home from school and found him the way I described him above
The water in the 10gallon i took directly from the dt, so I ruled out shock of any sort

I tested the water immediately, ammonia/nitrate/nitrite were all at 0, although the pH was a bit low (7.8 )
Temperature is at 76F, yesterday it was 78F but I guess the heater is weaker than in the dt
Also, the clownfish seem perfectly fine, so I really have NO CLUE what could be wrong or what to do

He has no external blemishes (no torn/rotted fin, spots, etc)

So what could it possibly be??????
IDEAS PLEASE anyone I rly love my fish and I don't wna lose him

 

[CARM]

ok I just noticed his tail is a little scruffy???

 

[Badfishofbabylon]

He might be stressed because of the small size of tank you have him in. It may just be temporary but the fish doesn't know that. Try turning the lights on the small tank off see if that helps out. Are the clowns beating on him?

 

[CARM]

he died yesterday.....and this morning I fed my 2 o. clownfish and there not eating...... iam freaking out are u sure it's the size of the tank that's affecting them? because for just stress the damsel went pretty fast.....

 

[Badfishofbabylon]

I'm not for sure, it could be a thousand different things, but I do know 10g is too small for 3 fish like that. How long till the clowns can go back to the bigger tank?

 

[Krypt]

I'd get your ammonia levels tested at your lfs just to be safe. The 10 gallon is not cycled and I'm betting it's ammonia poisoning. I know you stated your test reads 0, but depending on the age of the test kit and the type of test, it may be wrong.

 

[Badfishofbabylon]

Or the cycle may have not started yet when you tested it.