I totally missed temperature. You need to boost, slowly, until at around 78. While 75 is not lethal, it can stress their immune system and make them vulnerable to disease.
He actually appears to have fin rot. Can you get a photo of him from the side while he's swimming? And from above?
For now, take him off the Paraguard for at least 24 hours. Get SeaChem Kanaplex as it treats both gram negative and gram positive bacteria. Paraguard, if memory serves, only treats parasites and fungus. And if he's had a similar issue since you bought him it isn't working.
To heal wounds the best "medicine" is either Indian Almond Leaves or plain Rooibos Tea. Both have antifungal, antibacterial and calming properties. They can be used in connection with Kanaplex.
If floating a leaf or teabag doesn't achieve at least the tint below, brew a strong tea, condition and add to tank until color is achieved. Leftovers can be stored in the refrigerator.
Has he perked up at all since the increase in temperature?
When I have an ailing fish I feed frozen Bloodworms. You could try those.
These are great fish but they can be more fragile than many allow. Sometimes they sicken and we never know why. In 55 years of owning them I've had any number than left me scratching my head and feeling guilty.