Have you at any moment added water to your buckets ? If at any point you topped the buckets off with water you dilute the substance. In other words, your drop your PPMs. If you started with 300 and added Zero water and your PPMs are stable, your plants could be supplementing from the NUTRIENTS(lower ppm). Also the meter your using.... ? It could be that the meter is giving you another reading than before. It is best to get a PPM meter worth your hard work. If your meters great and your exercising its use properly, your plants are just feeding. It's a good thing. I'd say wait a bit before feeding. Try to get the PPMs to match almost nearly what the water started with before using nutes. EXAMPLE: my ppm READ 89 (tap water) after nutes it goes up. As the plants feeds the PPM drops, as the water reduces and the PPM increases, plants are drinking more WATER than feeding NUTRIENTS, I WATER TOP off, wait, PPM check, WATER, wait, PPM check, REPEAT if necessary , then NUTRIENT top off, I always check the PPM in between adding WATER to assure it's near where it started. It gives me an idea where the plant is as far as nutrients and when it's necessary to feed. I would post a photo but no uploads allowed