My experience is mostly with Freshwater Cyano, but I would guess the issue lies with your sand itself. If you decide to dose again, first physically remove all the Cyano you can, and then when you go to batch the chemical, move your sand bed around to ensure you can get the chemical to penetrate the bed.
If you don't get rid of all of the Cyano, it will reappear unless you fix the source of the issue. For freshwater, the solution for me was adding oxygen (Since it killed off alot of plant life and I assumed the plants left weren't providing enough O2 for a healthy balance), adjusting my CO2 injection, and having a blackout period, while dosing excel. My water parameters were always spot on for freshwater, and I never figured out what the source really was, but my actions to remedy it must have stopped the source.
My 2 cents.
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