I started up my 20 gal freshwater tank in September 2017. I let it cycle for a few weeks and tested my water before adding fish. All was going great until I had a sudden ammonia spike that killed 4-5 of my fish. I managed to get my ammonia levels back down to normal when I installed a new filter. Since the ammonia spike though, I have had constantly high nitrate levels. I've done multiple water changes,put Purigen in with my filter, feed the fish only what they can eat in 30 seconds or so and only give them Omega One food. I have also added a Hornwart plant to the tank and my nitrates are still consistently high. What else can I do?! I'd love to be able to restock my tank again someday and get my healthy water back.