Hi there everyone, looking for some help as a new guppy tank keeper. Running a 10 gallon tank with good filtration very clean Clearwater, been up for about two months and one guppy started behaving odd and has developed a black mark above its left eye. Currently have him in a quarantine tank and I’m treating with erythromycin, but no significant improvement and his eating is way down to almost nothing. All the other fish are happy and healthy. Parameters are all normal. Have added a small amount of aquarium salt to the main tank about 30 g per 10 gallons, just for general health.