Do the usual stuff with getting nutrients under control, increasing flow, and siphoning the cyanobacter. Sometimes after all that you can still have a problem. I think it may be because some of the cyano doesn't get removed and it dies off, only to regrow elsewhere in the tank.
If you still have a problem, you might try erythromycin. I had to do that on the wife's softie tank because she had been overfeeding her fish. I treated the tank maybe almost a year ago, and it has been cyano free ever since.
Here's directions that I pm'd to someone a while back:
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I'm thinking you might want to wait on the erythromycin because the red slime is just from the slight overfeeding and the skimmer being off. You might be able to get rid of the remaining cyano by siphoning it out and increasing the water flow. If that doesn't get rid of it, then try the erythromycin.
The brand we used was E.M. by Aquarium Pharmaceuticals. Very inexpensive compared to Red Slime remover and Chemiclean.
When using E.M., you dose at 1/3 strength. The directions say 1 tablet per 10 gallons, so you would use 1 tablet per 30 gallons of actual system volume. If you use just slightly more, it shouldn't be a problem. The erythromycin is safe for your biological filtratration even at full strength according to the product info.
If I remember right, the med affects mainly gram positive bacteria, and also gram negative that are facultative anaerobes. The gram negative non anaerobes which are your nitrogen cycle bacteria aren't affected. Cyano is gram negative anaerobe.
When treating your tank, you leave the skimmer off and run without carbon. Open up the capsules to dump in the med. That way you don't have the extra gelatin capsule dissolving in your tank. Put the med in your tank some place where the fish won't chomp on the particles. You could try to dissolve it in some tank water before dosing. I just put mine in the sump.
You do the treatment for 3-4 days straight. Prior to each successive treatment, you run your skimmer for an hour or so to pull out the dead dissolved slime. Your skimmer will pull a lot of water, so you'll probably have to adjust the air and/or water flow to skim a little drier. Make sure you're around when the skimmer is on, so you can make adjustments or top off the water as necessary. Turn off the skimmer when you're ready to dose.
After your last dose, just let the tank run without skimmer for the rest of the week. You should be seeing the cyano disappear by now, if not sooner. After 1 week, you can run your skimmer for about an hour each day to pull out the dissolved slime. You probably will need to gradually adjust your skimmer back to normal settings over the course of the second week. You could skim out about a gallon of slimey water each day.
2 weeks after the start of treatment, you should do a large water change, 40% or more. Then you can put your carbon back, and get your skimmer adjust back to it's normal settings if it's not there yet. Mainly, you don't want to reset your skimmer too fast, too soon, because you'll come home to find half your tank water skimmed out onto the floor.
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