It is not nothing by any means.
Your fire eel is covered in sores, big and small.
I don't observe any filter on your tank? Nor a strong bubbler? How is your tank set up?
Fish don't get sick for no reason. We keep the water right, the fish keep themselves and usually heal themselves in good water. Something in your setup and your husbandry is very wrong and we need to troubleshoot to find it.
A weekly "deep clean" too gravely concerns me. This may easily be a harmful practice.
Tests always near perfect? Post your water parameters in ppm, by an API test tube kit. Words will not help us help you, nor help your fish. I recommend using a test tube of known ammonia free water for reference against your ammonia test instead of the color chart, if the ammonia amount is 0.25 ppm or less.