Sounds like parameter swings wich new gfo will cause if adding too much. Gfo will greatly decrease your alk when freshly introduced. Happened to me recently and I lost most of my sps.
My issue was that my alk kit was reading 1.5 dkh too high and I thought I had it at 7.5 when really it was at 6 and then I introduced the gfo and it lowered it way more wich caused my corals to bleach.
Ive also had similar issues when raising calcium too fast.
Make sure your test kits are reading correctly and try to keep levels as stable as possible to prevent further damage.
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