Hi so I have an established 29 gal tank. It's been established for almost a year now. Ever since having my tank fully cycled I've never had an ammonia spike before. I usually do weekly water changes and about 70% water change. I've done this since my tank was first cycled. This morning all my fish seemed fine but when I got home at night one of my fish was laying on its side and some of the others were sluggish. I took a water sample and and immediately did a 75% water change. When I tested my ammonia it was at about 1 maybe 1.5 ppm. 0 Nitrites. About 20 ppm Nitrates. Any suggestions or reasons why this could happen???