mysterious fish losses

[Aquamarina]

Hello. I am becoming very concerned about the health of our aquarium, which we took over from the house's previous owners about 18 mos. ago. Up til April this year, everything seemed to be ok, except that the protein skimmer had not been working up to par for several months. We installed a new pump on May 14, and it has been functioning since.

April 27, yellow Tang found dead in tank. No outward signs of infection
(SG 1.022, nitrates 80, nitrites neg, Ammonia neg, pH 8.8)
May 24, ocellated clownfish found dead, no outward signs infection
(water test same results, except nitrates around 60--I have tried to reduce the amount fed)
May 29, pyjama cardinalfish obviously struggling, fins had been chewed on, died June 2
(On May 31, added Marine-Max, a disease prevention liquid)
Today, June 5, butterfly fish ill, fins nibbled on--moved to quarantine tank. Scales lighter in some areas, but no signs of white spots

The 10 fish and 2 stars remaining appear to be healthy and eat well. The tank is about 230 gal. capacity. The only odd thing I've noticed lately is that there is a white sediment left on the water surface and on the sides of the salt-water mixing tank. We use Instant Ocean salt. Feed is a variety of frozen cubes.

Any thoughts? Is there someone in this group that I could call to give me an on-site opinion in Bowness? As you may guess, our experience with salt water fish dates back only to the day we took possession of the house, so we still have a lot to learn!

Thanks.
Anne