fish gilling/breathing rapidly

I have a 54 gl corner tank which is a month old now. I have 1 blue
damsel and 1 three striped damsel, as well as 2 clown fish. I
introduced the damsels about a week in to simply get some amonia into
the tank and introduced the two clown fish in the past week.

The fish have always been gilling/breathing rapidly. All my levels
seem acceptable
PH - 8.5/8.6
Amonia - .25
Nitrite - 0
Salinity - 1.025
temp - 79
filter - Fluval 3050

I am running out of ideas, as to why the fish are gilling rapidly, and
as of this evening, one of the clown fish appears its going to die.
None of the fish have open wounds, colliflower pathes, or other
deformities. The clown is lying on the bottom of the tank, and
getting blown around by the powerhead output. From what I have read,
the distilled oxygen could be low, but I have no idea how to get
additional oxygen in the water, other than allow the powerheads O2
input to be totally exposed.

Any ideas as to why my fish appear to be breathing heavy or how I
might correct any issues with the sal****er?