Duncan and Sand sifting star

[metanis]

I am a little perplexed by my losses recently. My duncan coral which was doing well and even had some new heads suddenly died over the past week and my sand sifting star died today. I recently lost my leopard wrasse within the last month as well. I have a 110 with a 30 gallon sump with protein skimmer. I'm running MH with 2 supplemental T8s. Current inhabitants include: blue hippo tang 1.5 inches, 2 inch perc, 1.5 inch royal gramma. There are 2 BTAs and a hammer with 6-7 heads. I have been doing 20 gallon water changes weekly and my parameters 2 days ago were as follows:
Temp: 78.2
1.025
pH 8.2
Ammonia/Nitrites/Nitrates all 0, not sure how my nitrates are 0 but I have a really low overall bioload.
Phos 0.25
Ca 480
kH 125

I feed 6/7 days per week with a variety of pellet, mysis, brine, and nori flakes. My aiptasia and cyanobacteria are certainly thriving. I am thinking of backing off a bit on my water changes to biweekly as long as my nitrates stay low.
Duncan was at the bottom and may have been too close to some green plays. Any suggestions based on this information or am I on the right track with my maintenance?

 

[oscarinw]

Maybe a metal present in the water? Any submerged metals? If you suspect that, throw a poly filter in the sump and see how things react.

 

[metanis]

No metals that I can think of. I switched all of the ring clamps over to plastic when I got the tank and it's been set up for quite a while now. I will check the sump plumbing again to see if there is anything I may have missed. I'm re-checking parameters today and performing a water change. Fingers crossed!