Help, im at a loss. 5 fish dead in less than 1 week

[Hightower]

Ok. I cant figure this out. (90gal)

In the past week I have lost my hippo tang, kole tang, 2 clowns, and a green chromis. A bangai cardinal has a massive eye on one side but not dead- yet.

Hippo tang got ick, and couldnt fight it off. ( no I dont have an extra aquarium for quarantine) Im sure he gave my kole tang ich even though he was healthy and eating lots including garlic, he didnt make it either.

Next my two clowns died within days of each other. LFS thinks it may have been clownfish disease. And in between all that, my blue green chomis who was eating voraciously, died. Go figure that one. the easiest fish to keep.

Parameters: Ammonia, Nitrite, =0 Nitrate= 2.5
Ph=8.0 temp 79, salinity=1.024 Phosphate= .1

History:
tank is 5weeks old, however, there is 100lbs of live rock cured and established from a 1.5yr old tank transported within 5 min. I monitored levels during those weeks, there was barely any detection. So not much of a cycle to go through.

I stocked the tank with 1, maybe 2 fish /week. still no rise in levels.

A 20% water change was done 1 week before they all died, with RO/DI water. TDS =0

With my T4 pump, im getting about 1000-1100 gph, along with my ASM G2 skimmer.

Still cant figure out what im doing wrong. Is this "beginners luck"?
This is not great for a wife who was skeptical, and now sees this, especially after sinking all this money into it.

The only thing I have noticed in the last few days, is a rise in brown "dusty" algae. But phosphates remain at .1

That maybe a tad high, but a rowaphos sleeve is in there.

Any help would be appreciated.