buy 1 pen and dont waste more on soil meters, its the same thing and the probe ones suk. the ph will most likely be low in the medium. so need to set to right ph and run through till it comes up. dont adjust the ph to make the medium rise or fall. the soil ph and food ph are 2 diff readings also. keep ther medium about 6.2 to 6.7 and always use same ph for foods and water days.
your going to need ph up and ph down to work right, dont use baking soda to adjust ph up as it kills off microbial life in soil