Please Help

This is long, apologies in advanced. We have a 265 gallon aquarium with sump, protein skimmer, wave makers, three 400 watt metal halide, sand bed and 280 lbs of love rock. It has been set up for 8 years. Had a mean powder brown kill almost everything before she finally died. Left with breeding pair of 20 year old cardinal fish, coral beauty 8 year old, and big pink spotted watchman goby. Began to restock tank, added, two kole tangs, five yellow tangs, breeding pair of ocellaris, purple tang powder brown tand, rose bta, green bta, fuzzy mushroom coral and green mushroom coral. Took water sample to lfs when we lost a kole tang, everything perfect but phosphates. Bought a ro/di filter, began making water, then all of a sudden the other kole, two yellows and purple died. Well the powder brown brought in oodinium. Pulled the rock, the only remaining fish, one ocellaris, one yellow, one sailfin, the stupid goby and one cardinal, they are in 55 gallon hospital take treated with copper doing well but the cardinal passed. Rock went in to water to rid phosphates, anemones and corals stayed in take. 100 gallon water change with to water, added two bags of dry sand, replaced rocks. Now my corals are doing great my anemones are about dead. Phosphates are gone, ph 8.4, ammonia 0, nitrite 0, salinity 1.024, nitrates 5 ppm too high but coming down. What can I do? Hermits coral banded shrimp and snails are fine.