I don’t think this has anything to do with the algae. Even if the algae came from a disease infected tank, the fish died two days later. Not really long enough time for the macro to be the cause.
Any serious toxins to kill fish would have killed the inverts too.
It’s not going to mess up your oxygen either.
It’s probably a coincidence and the fish already had a parasite and they happened to die two days after.
Do you have any rock in there? I mean reef rock sold as dry or live. I am wondering if you are going to have enough biological filtration.
I see lots of fake decor. Sometimes they are not appropriate for saltwater environment as it is a much harsher environment than fresh.
When were the fish added?