Antenna catfish with strange white spot and side fin

You need to stop feeding your fish for some time while you wait for the bacteria to grow to a level where they can handle the ammonia and nitrite in your tank.

To lower the danger of nitrite poisoning you can add aquarium salt so the concentration is at 0,3% salt in your tank (most fish will handle this concentration of salt for a long time).

You need to change about 20% of the water today to get the level of toxins down (remember to use a dechlorinator when you change water).

Take daily water tests until the level of nitrite starts to go down. In a tank that is properly cycled, this does not take long.

Next time you clean your filter be sure not to change more than 30-50% of the filter media to keep the biological cyclus going in your tank. You do not need to clean the filter until you can see the water flow is reduced.

If you have filter foam/sponges in your filter, you only need to rinse it. Be sure to use water from your tank when you rinse the foam to avoid chlorine killing the bacteria.

Your fish looks like it has got an fungal infection. If possible, treat your fish in a separate tank. This is to avoid medicating the main tank, as this will kill the nitrifying bacteria and cause problems for the rest of your fish.