Should not flush unless necessary, if you do it needs to be thorough. Nutrients are mobile/immobile, nitrogen has different types within the soil, one that's + and one that's - charged. One is highly noble in soil, the other is highly immobile in soil, when you flush if you don't do it thoroughly you can leave the ammoniacal nitrogen immobile in soil, washing out all the mobile nitrates and nitrites. If ammoniacal goes above 20% you will have problems. Make sure you know what nitrogen you have in your soil.
If it was phosphorus I'd expect purple stems or purple something, none of the symptoms give p itself is mobile and would present old growth first of it was deficent. It looks to me like it's all over the plant, which would indicate to me it's more of a pH skew affecting the entire plant.
Lime green interveinal chlorosis, where leaves are yellow with green veins, is most often caused by a deficiency in iron or manganese.
Iron is crucial for chlorophyll production and enzyme activity. A deficiency leads to interveinal chlorosis, where the tissue between this is often due to high soil pH, Higher soil pH levels, the availability of most micronutrients (like iron, manganese, boron, zinc, and copper) decrease.
Need a pH of the medium first and foremost.