Cyano loves low flow area's, increase the flow if you think you have low flow.
If you use hang on filter, they do not really export nutrients or dissolved organics, so water changes are the only method of removing nutrients and organics, I would increase the frequency and amount of water changes until your tank gets back into balance. Water change every 2 weeks is the bare minimum, but when your tank has a problem you should do more.
Chemiclean works very well, so if you tried it twice and cyano came back both times, then the conditions in your tank are very favourable for cyano growth, need to correct that.