Fish are acting very weird (2 of them might be dying)

Hi,

I have a 20 gallon freshwater tank. In this tank i have the following:

4 Tiger Barbs
4 Black Widdow Tetra
1 Diamond Scale Tetra
2 Serpae Tetra
1 Snakeskin Gourami
1 Red-Tail Black Shark
1 5-6 inch Common Pleco (the pleco is about to be given away to a
friend).

The tank is about a year old, fully cycled. About a month and a half
ago, I had a very large rise in phosfates. A freind of mine
reccomended I put in a"phos-zorb" filter medium bag. He said that I
should take out the filter pad, and insert the phos-zorb where the
filter would go. Earlier today while feeding, I noticed that 2 of the
Tiger Barbs were hovering at the top, and their mouths were going very
fast. I thought maybe there was insufficient amounts of airation in
the tank. SO i stuck in an airpumpm. Even with the airpump, they are
still gasping at the surface. Also, upon closer inspection, it looks
as though these two fish have a couple ich spots on them I had some
ich medicine left over from an ich spell i had about 9 months ago, so
i added the recommended dose. I just dont understand what I am doing
wrong. My Ph is at 6.7, my ammonia is at 0, my nitrites are also at 0.
AND I have an air pum going, in addition to the filter. Can someone
please help me? Thanx for reading

Have a good day!
Evan