Good info Tom...I had a serious cyano issue in our 14g. Had GFO, Chaeto, carbon and CPE running in it. After someone suggested removing the CPE, after it had been in tank for a month plus, I have had no cyano. I def. believe it was leeching back into system.
Edit: Granted I changed my husbandry practices significantly so that maybe part of it but it was only a cpl days after removing the Chemi, so I don't feel it was coincidental.