fish death

I've been plagued with poor fish survival in my tank, over the past year
and a half i have lost 6 clown fish, some within the first 3 days, the rest
within two weeks, ive also lost two blue damsels, and apparently today while
i was acclimating two clowns [last try before i scrub the entire tank] my
bicolor dottyback decided he had enough of the tank and jumped out. I've
been racking my brain on whats been going wrong with my tank, nitrogen
levels, not enough disolved oxygen, acidic ph, but when i do my tests
everythings within norm, even temperatures have been steady at 78 degrees.
If anyone has a clue as to whats going wrong, feel free to comment.

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75 gal tank
30-40 lbs live rock
80 lbs live sand
1200 powerhead
250 watt titanium heater
150 watt glass heater
1500 Cascade canister filter with pads and 1 liter Matrix bio-media
protein skimmer

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1 emerald crab
several blue legged hermits
one turbo snail
2 astrea snails
1 bi-color dottyback [if its still alive]
2 common clown fish


If these two clowns dont make it, the next step is to tear down the tank and
put it in storage.