I would get one of those Koralia powerheads. For a 39, I would grab a #2 or a #3. I used to have some cyano issues and although I can't give all the credit to those powerheads, they certainly helped. Check your phosphates as well... I had to replace my DSB, because it was essentially saturated. The combination of the two (more flow and eliminating phosphates) has wiped out the cyano.