A mass die off of cyano
(or any algae for that matter) can cause ammonia to spike. I'd remove as much as you can first manually and monitor water quality. If you go ahead and get rid of it now, much won't die off.
As with most other things, erythromycin is sorta getting rid of the cyano
, but not the cause of it. So IF it comes back, it will be with a vengence, will have more than likely altered in some way, and will be harder to eradicate the second time around. That's why using antibiotics to get rid of it isn't reccomended.
So that said...
Have you tested for nitrate and phosphate? How much flow is in the tank, and is it reaching the areas that are being taken over by the cyano
? How long are you lights on per day? What do you feed, and how often?
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