I have a 20 gallon high tank with 2 guppies, 2 platies, and a molly. I've had this tank for probably about 2-3 months. When I first got it the water tested 0 on everything and then slowly some ammonia showed up, and one time I tested there were some nitrites (debatable though I'm not sure the color was actually reading nitrites) but I've never had any show up after that. Eventually, I tested positive for nitrates probably a month after that (I hadn't been shaking bottle #2 so that contributed). I thought my tank was finally cycled. I've been testing it about once or twice a week since and it shows .25 ppm Ammonia and 5 ppm Nitrates every time. The fish seem completely fine as well, with the exception of one that died of fin rot. I did a 100% water change immediately after removing him from the tank and none have shown any problems. However, I tested the water yesterday and there was a giant ammonia spike like I've never seen in my tank. 1 ppm ammonia, which was odd because my fish weren't acting lethargic or poisoned at all and I don't leave food in there to decay. I performed a 25% water change and dosed with Prime just to be sure my fish would be ok. I tested again this morning and the ammonia was at the same level as before the water change and the Nitrates were still at 5. I did some research and saw someone suggest that the tap water be tested because the nitrates may be from that, and not the cycle. Sure enough, my water's ambient Nitrate level is 5 (the tap water also is 0 for ammonia). How should I proceed from here? Do I need to get some bacteria to put in and treat this like a fish-in cycle? I'm attaching a photo of the tank for reference.