The filter itself doesn't primarily remove the harmful toxins noted as ammonia and nitrites. Nitrates are also harmful but only in elevated amounts. Nitrate poisoning is somewhat a rare occasion, especialllly in uncycled tanks..........filters don't actually clean the water of any ammonia, or nitrites residues, at all. What is contained inside them is what oxidizes the harmful substances into less harmful compounds, nitrifying bacteria grows in places with a regular supply of ammonia/nitrites and lots of waste/decay. They will thrive in sewage areas where there are exceedingly high amounts of waste and organic decay.........ammonia monooxygenase which converts ammonia (NH3) to hydroxylamine (H3NO) , and nitrite oxidoreductase, which oxidizes nitrite to nitrate Are the main concerns with keeping your ammonia levels minimal.........
Carbon and charcoal have virtually no need in the home aquaria but for uncycled tanks, some people will use zeolite, which many people confuse the purpose of with carbon......to eliminate ammonia.....there is virtually no need for these and you don't need them
Yes, you do add water conditioner to a non filtered tank as heavy metals. Chlorine, and chloramines are all toxic to fish and will have a negative impact on them. Resulting in likely stress and death