I have an EC meter, but I never use it, for the reason that it can be unreliable until the plant grows roots over the entire pot, and that's when it starts to stretch and bloom. The best indicator is the tips of the leaves, when they are slightly burnt, it means that the soil is saturated nutrients, if the new leaves that grow have burnt tips, then you do not need to fertilize the soil and it is enough to feed it again in a week, if the tips are not burnt, add a little bit of fertilizer with each watering until you notice that within two days after watering, a change on the tips of the leaves, that way the roots will always have food available, but you have to be careful and with all the watering you have to pass 20% of the run-off water through the pot.