High Nitrates

I have an in-ground, plaster pool, about 26000 gallons, cartridge filter, and I use chlorine tabs. My pool store tested water and indicated that my nitrates are 30 ppm and causing a loss in chlorine. I have a very small amount of algea but the water is clear. My chlorine levels right now are very low, both total and available. My CYA is high at 100 PPM. PH is 7.8, alkalinity is 110 ppm. Calcium hardness is 300 ppm and TDS is 800. Their suggestion is to drain half the pool to get rid of the nitrates. We are in an extreme drought right now, and I dont really want to waste the water. Other posts suggest that adding a large amount of liquid chlorine might help. Any other suggestions? Once the drought is over, I can drain and refill as needed, but want to avoid it if i can.
Thank you.