How are you testing your water (kits) and Can you post pics of your tank under white lighting ?
Disregard nitrites unless they are sky high. Nitrates are elevated pointing to water and filter. I see you mentioned HOB filter. Part of your issue is lack of filtration. Your hang on unit is mechanical filtration and you want to have mechanical , biological and chemical to manage proper water quality. The HOB will polish water and trap particles via the cartridge.
Chemical is what traps and breaks down chemical compounds such as feces and uneaten food often accomplished carbon and GFO
Biological is what utilizes the natural process of iological filtration such as use of ceramic nuggets, bio blocks, and microscopic bacteria surfaces.
Ammonia spikes may have beenvery possible and often undetectable after the spike event. At minimum, add a hang on Skimmer unit such as Ice Cap K1-50 or K1-100 to remove inorganic waste and adding oxygen![]()
As for green water. . . Is your tank at or near a window? This time of the year, UV rays gain intensity and will send a tank green in a hurry