Agree, sound like cyno.
Just to add a few things. You need to check you water to see what the PO4
level is. Let us know what the level it. Then the following can be addressed.
1. Do you use RO
water? If not your should make the switch and work on doing some water changes. 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 the cyno 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.Like malkore asked, 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 in the short run, but you need to address the other underlying causes.
These are few of the key things that feed a cyno outbreak.