Well, as you already now, cyano
is typically caused by poor water flow and excess nutrients. I wouldn't think the vitamins could be causing this unless you were really overdoing it.
Do you have any kind of nutrient export other than the skimmer? Any macro algae? Also water changes do help quite a bit with nutrient control. I change 20% every two weeks. You should definitely carry out that 25% you are talking about.
What kind of water are you using? If it is not RO
, that would be the majority of the problem. Also, do you have enough water flow in the tank.? The more the better as cyano