so the tools that you are talking about would definitely give you a better advantage. A PPM meter will tell you how heavy a feeding you are giving the plant. similar to a caloric intake for humans. and you also say you are using tap water. apparently you have some pretty fair tap water but it is a good idea to KNOW where your pH is and measure it. Alot of growers will go off and buy the cheap little $20 pH meter and they can work ok. i really good reliable one should run you about $100. 

An alternative to a pH meter is using pH test drops, which is what i use.  (the advantage of pH drops is that they NEVER require calibration so they are almost NEVER wrong) Also, they are cheap and last a good while

Lastly since you say you are not feeing the plants any nutrients, i believe that it has grown so large that the soil can no longer sustain it and needs at least CalMag and some finishing nutrients like a M.O.A.B or ChaChing

i hope that helps (more soil growers will add to this)

Peace