In your situation there are two essential things you need. You need to know the real level of ammonia and nitrite(NO2) in your aquarium, and the minimum tool to do that is a good liquid-reagent based test kit (like the API Freshwater Master Test Kit, which many of us use) and if either ammonia or nitrite is at 0.25ppm or above then you need to take better action about that with proper water changes with good water change technique. The problem is that when the toxins are above this level then this concern outweighs nearly all others. The remedy, being water changes, is simple, if laborious, but the problem is that it is often not fully understood or is put off.
Small amounts of ammonia are causing permanent gill damage and small amounts of nitrite(NO2) are causing suffocation and permanent nerve damage. The fish can feel this and to them the stress will be dire, even though they just look like fish sitting in water to our eyes.
Having sounded alarmist, let me say I don't mean to sound that way, and I'd like to welcome you to the forum Keeks and I wish you the best of luck with your mollie or whatever the black fish turns out to be! The great members here will keep you going with lots of support.
~~waterdrop~~