Here is my standard post for algae or cyno control and elimination.
1. If you already use RO
water, test your source water. You may need to add a deionizer to it. This is the most important step.
2. Siphon as much of it as you can. Breaking it up only helps it to spread. You need to do this as frequently as possible.
3. Review your feeding habit. Are you overfeeding? Do the fish eat what you put in at feeding time within a few minutes, or is there food lying around the bottom after? Also, if you use frozen, make sure to drain the as much of the packing juice as you can before adding.
4. Do you have adequate water movement? Any dead spaces in the tank? Cyno has a harder time setting up in a system with brisk water movement.
5.Do you have a skimmer? If so, is it producing daily amount of dark thick "skimmage'? Adding or upgrading a skimmer may help.
You can also run a PO4
sponge which will help, but you need to address the other underlying causes.
These are few of the key things that feed a cyno outbreak.