Why did you treat the tank with copper?
While it's possible to remove copper with Cupresorb, as you're experiencing it's not easy. Given that this looks like a smaller tank, it might be worth just starting over with new rocks and sand. The tank itself should not bind copper and will be fine otherwise reuse.
I would also be concerned that your nitrate and phosphate are quite low, and I would suggest running them higher, despite what you may have been told or saw in the past. Running zero to very low nitrate and/or phosphate invites dinoflagellate algae, which may be what you're dealing with.
I follow the guidelines from Randy Holmes-Farley on parameter levels found in this post:
https://www.reef2reef.com/ams/randy’s-thoughts-on-nutrient-target-ranges.1087/
From that link:
- Let nitrate float between 5 ppm and 50 ppm. I’d use gentle export in this range, such as growing macroalgae.
- Above 50 ppm, I’d begin to focus more on reducing it, by organic carbon dosing, turf or macroalgae, etc.
- Below 5 ppm, I’d begin to dose ammonia or feed more. The target level might drop lower if dosing ammonia, just like the heavy in/heavy out scenario where nitrate may not be as needed.
- Let phosphate float between about 0.06 ppm and 0.3 ppm. This range is higher than I’ve recommended in the past. I’d use gentle export in this range, such as growing macroalgae.
- Above about 0.3 ppm, I’d begin to focus more on reducing it, by turf or macroalgae, or a binder such as GFO or lanthanum (has its own risks to tangs). If a binder: GO SLOW. Turf and macroalgae will typically be slow enough.
- Below 0.06 ppm, I’d begin to dose sodium phosphate or feed more to get the level up.
I hope this helps and good luck!
