ppm meter? thats not necesary if ya can read what ur plants are telling ya.
i have never had one and i will never buy one.
i use canna coco nutes - a very complete set of nutes for coco-growing. there is normally never deficiancies as long as the PH is controlled.
coco coir and fibers are like a sphonge really - they got a really high water contaiment capability.
im watering my 5 gallon pots with about 4-5 liters every 1-2 days but i know they could hang on a while more with the humidity in there - but i feel better of watering and feeding them well. ive been now for years handwatering - but i finally decided to get a pump and set it up automatic since work is taking too much time and the babies are suffering at times.
my set of nutes:
canna coco A+B
B'Cuzz root juice
revegged SSH x PTK and PTK/SCBB x PTK - yeah - thats about 6 x 6 feet and one 600W.
this is the same bushes a few weeks before.
my best aditive - the one i think i allmost cant be without is - vitazyme - its like plant steroids. gives an enormous boost to health and growth rate.