It's perfectly fine to refill your reservoir if your plant empties it before the growing season is done. Warm weather makes for thirsty plants that substantially drop the water level in the tank. You may also have just misjudged how much you needed or wound up with a real grower of a plant stripping the nutrient level. If you do refill you need to be aware of the oxygen needs of the root system. You can't just fill the reservoir to the top as you did at the start. Aim to cover about half of the total root mass. This will keep the nutrient level just right while providing roots the air they need to thrive. You also need to really check the EC and PH. High temperatures can also cause your solution to become too concentrated. It pays to test before you close that lid again.