Corals bleaching/closed before water changes

[Stephers]

I've had this problem for a while now, but it hasn't always been a problem. Like clockwork, 2 weeks after a water change, many of my corals start looking like crap. I have lots of different softies, Lps, sps. Lps are pretty unaffected. Acans and euphyllia are super happy. Almost all my zoas close up, half my montis start to bleach and stn but not all the montis. Acros are fine... I'm so baffled by this. After a 20% water change, everything will make an instant comeback and then start to decline again 2 weeks later until another water change.

My water stats are stable at:

Ph: 8.3 during the day
Kh: 7.5
Nitrate: 3 ppm
Phos: 0.02 ppm
Calcium: 420
Mag: 1400

It's a 75g with sump, carbon running. Ro water, cartridges changed regularly. Coming out 0 tds. Tank is 1.5 years old. I only dose phos, nitrate, calcium, and mag to keep the above stats stable. I do need to dose phosphates pretty much daily to keep it above 0 ppb.

I've done a couple icp tests which have showed nothing remarkable.

This is really putting a damper on the hobby for me. I've had reefs for 13 years and never had a problem like this. It's very frustrating.

Tank is looking really rough right now because it's been a month without a water change. I had a baby 2 weeks ago and have not been able to change water. The 3 montis that become affected have had a lot of die off. Hoping to do a change tomorrow and I'm sure everything will be happy for 2 weeks again...

Any ideas? please!

 

[Zero Nitrates]

Weekly water changes.

 

[Stephers]

I know water changes are solving it short term... but I can't just do weekly water changes on a 75g forever. It's really unfortunate to have to be tied to that schedule or to suffer large stn problems. There's obviously something bigger going on.