Nutrient Solutions
Hydroponic plants get everything from the water, so you need a complete hydroponic nutrient - usually sold as a two-part concentrate you mix to a target strength measured with an EC or TDS meter. Leafy greens want a weaker solution than fruiting crops. Check and adjust pH to roughly 5.5 to 6.5 so nutrients stay available, top up with plain water as the level drops, and replace the whole solution every couple of weeks.