I have four 6 inch gold fish, three betta fish share a 55 gallon tank with a canister filter and sponge filter for two years now. Recently I was frustrated with the nitrate level being high no matter how often I changed water. The nitrate keeps around at least 80ppm if not higher. So I did a 75% water change. A week later my fish started to act weird. One developed black patches on it, another one today is hanging head up and looking like forgot to breathehttps://photos.app.goo.gl/nUKP2rJmr9PWxf6E8. What should I do? Leave them alone? Add salt, meds?