I have all my probes fully submerged in the sump, hanging on the side of the glass with the old aquatronica probe holders or magnetic clips holdin them. They (ph,temp,orp,) works fine. I also have 1 temp probe just lying in one of the external overflows just in case the return pump fails I can notice the temp difference between the 2 temp probes because all the heaters are in the sump. I do clean the probes in the sump every 2 weeks with a soft tooth brush.