And jump starting a tank with old filter media debris doesn't cycle the tank all it does it add starter bacteria to the tank you added too many fish at once. The bacteria needs to grow enough first so it can handle the load in the tank.
It sounds like the tank isn't cycled you should watch this video about the nitrogen cycle.
To combat this you are going to need to do daily water changes until the tank is cycled you should also get nitrate test kits and test the water every few days.
I heat the whole room I don't use tank heaters but what I do is fill a 5 gallon bucket and let it sit for like 20 minutes then add it to the tank. But I use sponge filters so they are always under water not HOB filters it could kill the bacteria if you do it this way.
One trick I found when removing visible parasites is hydrogen peroxide I take a q tip and soak it with peroxide and rub it on the parasite most of the time this will agitate it and it will release from the fish but if that doesn't work just remove as much as you can.
If it only has one then it may have come in with substrate or a plant or something but the good news is you can simply net the fish and take a pair of tweezers and pull it off. After that I would wait and see if more come back before you treat the whole tank as it might not be needed.
You can keep aggressive fish with large koi but I have fished these before and they are like bass and attack anything that moves if they are hungry or in hunting mode. From what I have seen if the fish can fit in a predators mouth they will eat it or try. Also chain pickerel get huge and will...
Males have a thin analfin females have a large fanned type fin.
Top is a male bottom is a female from the picture it looks like you have a female mainly based on the shape and color I can't really see the fin. Also it doesn't look pregnant just fat/stubby.