As far as a formula to convert from conductivity (mS or uS), I don't know of any simple one... Siemens are the base unit of conductivity. In the case of water, conductivity is a measure of total dissolved salts (TDS), which is directly related to hardness (the higher your TDS, the higher the hardness). mS is milli-Siemens and uS is micro-Siemens. There are 1000 uS in one mS. 0.00 to 4.00 mS (0 to 4000 uS in with 10uS accuracy) should be quite sufficient for fish keeping. Hope this helps, --Cliff ------------------------------------------------------------------------- This is the apistogramma mailing list, apisto@majordomo.pobox.com. For instructions on how to subscribe or unsubscribe or get help, email apisto-request@majordomo.pobox.com. Search http://altavista.digital.com for "Apistogramma Mailing List Archives"!