Hi! I am new to fish keeping but have been trying my best. I purchased a “precycled” tank about 2 months ago. AKA it was cycled with fish in it already at the store. They packed it up, water, fish, plants, everything and sent me home to put it all back together. Well, it was a poor impulse purchase. I have since learned found out it wasn’t cycled and it was overstocked. It is an 8 gallon and had 12 male endlers guppies and a betta fish. The betta fish developed fuzzy patches and died in less than 5 days. I felt terrible for the fish and responsible for not doing my homework. The tank was not cycled when I brought a water sample to the store looking for answers. I got an API freshwater test kit, treated the display tank for fungus as well even though I had isolated the betta to a hospital tank fairly quickly after the patches appeared, did a LOT of water changes, and got the tank cycled after a several weeks. It has been all going well since until 2 weeks ago I noticed I was only counting 11 endlers instead of 12. The tank is heavily planted with lots of rock caves so I thought maybe I was just missing one as I never saw a body. Then today after 6 weeks of 0 ammonia, and 0 nitrites, and 20-ish nitrates I have a spike of ammonia to between 0.25 - 0.5. Nitrites are still 0. Nitrates seems a little darker than normal but not all the way to 40. I pulled all the plants (they are all anubias glued to rocks- no actual planted) and rocks caves out today, looked for a body (none found but also confirmed only 11 living fish) vacuumed the sand well and did a 50% water change. I usually vacuum the substrate I can reach at each change but this was the first time I removed all the rocks to get underneath them. I plan to recheck the parameters in 2-3 days. Is this enough? I did rinse my filter sponge and bio balls last week but it was in old tank water I had just removed with my water change as is advised. I basically just swished them in the bucket of water I had just removed and put them back in the filter. My filtration system has a pump with 2 sponges, a TON of bio balls and I have a bag of chemipure green in there. Did I wreck my cycle unknowingly somehow? Or do you think this is from the dead guppy that must have been eaten/degraded? I don’t want to freak out and do too much but I also don’t want to end up with more suffering fish. They all seem quite normal right now. The are all active- super excited and come up to the glass when I am near the tank, eating etc, so I’m not really sure why one died. It just wasn’t there one day. I treat my new water with fritzguard and have been adding fritz 360 every 2 weeks. I will attach my records of the water parameters since the beginning, water changes, and stuff used below. If anyone has an opinion or advice, I would appreciate. My husband really wants to add back another betta or some Cherry shrimp but I told him I’m not ready for that and I think it would overstocking. I’m just trying to keep the ones I have healthy and happy at this point. Thanks in advance.