Start by doing a 30% partial water change with salt water that has mixed overnight (24 hours is better). Do another 30% PWC
two days later (if not the next day). That regimen might stem the deaths while giving your fish what they need (clean living conditions).
It sounds like the lfs
sold you the tank, had you fill it and then add the damsels to cycle the tank -- BAD ADVICE.
then continued to sell you fish, adding to the bioload faster than the bacterial population could grow to accommodate it.
New fish should be added at the rate of 1 - 2 per month.
What test kits are you using to test your water parameters?
Do you have a refractometer for testing SG
or are you using a swing arm hydrometer?
If you don't have a refractometer, get one.
Here's my rules for a successfully aquarium:
Rule # 1. Nothing Good ever happens Fast in a salt water aquarium
Rule # 2. Don't add anything (supplements/chemicals/minerals)you don't test for first.
Rule # 3. PWC
(Partial Water Changes) are your friend, and cure many ills.
Rule # 4. QT EVERYTHING before adding it to your tank (mandarins excepted, corals should be dipped)
Rule # 5. Use only RO
water (either buy it or make it)
Rule # 6. All animal species live longer on a 30% reduced caloric intake. Only feed every other day at most (fish species dependent)
Rule # 7. Hyposalinity is the best, safest, and most effective treatment for marine Ich (IMHO
Rule # 8. A Refractometer is a MUST HAVE, not a luxury.
Rule # 9. Anemones will never live anywhere near their normal (approx 30 years) lifespan in a home aquarium. Leave them in the ocean.
Rule # 10. The tank size recommendations for fish are there for a reason - HEED THEM!
I have been asked to add
RULE 0 - Don't believe the LFS