Questions - Need some input

Question? When ph probes start losing their calibration, which way do they shift the readings to? Do they read higher or lower? Maybe they are all different?

I wanted to get people's thoughts on using chemi-clean to get rid of red slime. I have used it in the past but it seems very hard on SPS coral. The slime is not all that bad (YET) but it is getting on my nerves!!! Let me know what you guys think!