Cyano Question

I had a bad breakout of Cyano and scrubbed it off of the rock,s and done two fifty percent water changes over the
course of 4 day's. I bumped my nitrates and phosphates down to 0 because I was told this was what was feeding Cyano. It is slowly coming back on the rock, Why? Should it not kill itself out?

The source was a rock from the lfs and the rock was scrubbed clean. Should I just pitch the rock?
I have ordered chemical's but want to avoid them?

 

try putting your lights on for shorter periods of time and increase the flow in those areas worst effected, i just used red slime remover in my tank, the cyano has gone now 3rd day, battled it for months, will never use it again though even the safest chemicals seem to be no good in my tank, it looks like my urchin has had chemotherapy treatment and all his spines have fell out,
Also make sure you have a good skimmer, its the number one at battleing cyano...

 

rock is leaching phosphates it sounds like