I came home to find two fish struggling to swim, they really werent swimming. They were floating face to the surface tail to the bottom of the tank and they never turned or corrected. It looked like there back fin just stopped working completely and they where floating straight up and down, I rushed to check the water parameters and they came out 0 nitrites .25 ammonia and nitrates at about 15. I just cleaned the filter and did a water change several days ago I'm not sure what happened. Any ideas?