High nitrates

I have a 55gallon planted community tank with 6corydoras 6black neon tetras and 5 Buenos aries tetras. I have had this tank for a couple years now and had been perfect. I am wanting to get more fish but have waited 2 weeks but have waited because recently my nitrates only have shot up to 75ppm. No matter what I do they won’t go down. I cleaned gravel, I’ve been doing water changes, I added more plants, I started feeding less, and I cleaned the filter no matter what I do it won’t go down please hel