help, fish dying

so about 4 days ago my lawn mower blenny went missing... i am now assuming he is dead, and then i came home today and found that one of my damsels was dead, and the other 2 were laying on there side on the sand gasping. i cant figure out what went wrong. my parameters are perfect.

0 nitrate
0 nitrite
the PH is 8.0
there is no ammonia present.

and im positive they are getting air because i have a 300gph water fall style filter that makes tons of bubbles when it hits the water's surface.

what could be wrong?

 

what size tank is this on?

 

its a 55 gallon

 

every thing has been fine up untill today.... i cant figure it out..

 

I assume the tank has cycled and your not using test strips?

 

maybe add another power head?

 

there are 3 power heads. two K2's and one K1, a 300 gph filter, and a maxi-jet 1200 running my bak-pak 2, the tanks is cycled and is almost 3 to 4 months old. im not using test strips im using the API reef master and the API saltwater kit's.

 

have you used any kinds of spray close to the tank?
like maybe putting on deodorant or something? air freshener?
are you running carbon?
is there any parasitic signs on your fish? maybe velvet?

 

the only thing spray wise was some cleaner on the out side of the glass, and i try really hard not to spray near the top, and that was a couple days ago. the only think i have noticed, is there are getting white spots. and looking very pale.

Im running carbon in the regular filter

 

white spots are usually signs of ich or velvet..
are they grain sized spots?

when you noticed the spots, how long till they died?