In promix i run calmag after a few weeks. With soiless grows ill mix a batch of full synthetic feed in a rez @ ppms that are corresponding with the plant size. Soiless grows are like a passive hydro in a way. Its drain to waste that needs constant feed. Just keep scooping out of the rez and dont worry about adding straight water ever. Make sure you have a ppm meter and dont over feed. Ppm meters use the base ppm of your water, which is usually between 100-200 ppms, and then food on top of that...+ a calmag supplement, all under the target ppms. So a plant like that looks like it would need around a 350-450 ppms. So not much food, but all the time. If it was healthier it could take more. Lots of people think soiless is like potting soil, packed full of food and all you need is water for months, but it acts like a water grow where it needs feed to flower. If you have another seed or two, plant it, get em goin now, because that plant is not going to make you a bag.