The problem is the coins nowadays would not be pure copper even if that method works. They comprise other heavy metals. Luckily, heavy metals are not as toxic in hard alkaline water as they are in soft acidic water but it's not something you want to risk anyway if you involve other heavy metals which might lead to whatever health issues to your fish that they can.
Even if the algae is destroyed, where does it go? When it undergoes decomposition state, it can use up oxygen and elevate ammonia. A lot of saltwater fish and inverts are very sensitive to that. Ammonia is more toxic at high pH and granted, saltwater does have very high pH at that.
Algae killing chemicals are not something to risk for in saltwater setups.
Even if the algae is destroyed, where does it go? When it undergoes decomposition state, it can use up oxygen and elevate ammonia. A lot of saltwater fish and inverts are very sensitive to that. Ammonia is more toxic at high pH and granted, saltwater does have very high pH at that.
Algae killing chemicals are not something to risk for in saltwater setups.
