[leebca]

I'd be concerned you had good enough circulation. A 75 gallon reef should be getting about 15+ turnovers per hour. That is, more than 1100 gph. If you have a sump and refugium, even more gph would be expected.

I would implement a quarantine procedure on all wet life going into the system. These are good reads on that if you care to do some reading:
An Ounce of Prevention is Worth a Pound of Cure: A Quarantine Tank for Everything by Steven Pro - Reefkeeping.com
A Quarantine Procedure

As Woodstock suggested. . .Get all fish out of tank into a hospital tank for close(r) observation. Let the main tank go fishless no less than 6 weeks. Then I'd be interested to know:
The sizes of the living and dead fishes?
How old is your system? When did it originally cycle?
Does the gallonage (75) include the volume from the sump and any refugium?
What kind of substrate (or none) are you using? How thick?
Do you use carbon, skimmer, mechanical or other chemical filtration?.
Foods you use and feeding schedules.
How are you acclimating the fishes – what procedure?
Where are you getting your fish and marine life from? (online, LFS, friend, or ?)
Do you use any vitamins? Fat additives? Any elemental or other additives? Please list all.
Water parameters – please give actual numbers (salinity or sp. gr. & range, temperature range)
Do you see any of the following in your system: hair algae; micro algae; cyanobacteria growths (red slime algae); dinoflagellate (zooxanthellae) growths; brown algae; diatom growth; slimes; off-colored patches on rock or substrate that are not coralline; etc.?
Water changes (how much and how often).
What is your source water? (Tap water, RO water, DI water, RO/DI, distilled, etc.)
List what you added or taken out of your aquarium system (living, decorations, and equipment) during the past 6 weeks.
Maintenance schedule. What have you done lately?

Fishes can die from an 'off' acclimation up to 6 months after getting in your aquarium. It is more important that most people think. This is the acclimation procedure I use:
Fish Acclimation Procedure