Betta- silver/blue with black streaks fin/body rot

[Carl]

This appears to be a stubborn case of Fin rot (generally a gram negative infection, commonly caused by aeromonas bacterium)

I would start with the article I provided in my last post: "How to Treat Sick Fish"
www.aquarium-pond-answers.com/2016/09/how-to-treat-sick-fish.html

As well as this one for disease prevention:
www.americanaquariumproducts.com/Aquarium_Disease.html

I can tell you that Betta Fix (same product as Melafix) is a gram positive mild treatment similar to using Bactine in humans for anything more than a minor scrape or injury, in other words it will not work for a full blown fin infection with is usually gram negative.
Reference:
www.americanaquariumproducts.com/AquariumMedication4.html#melafix

Ditto the Lifeguard product, while better, still not specific to this problem

this article more specifically addresses how best to treat your fish' issues, assuming you follow all the points in the article "How to Treat Sick Fish"
www.aquarium-pond-answers.com/2011/06/betta-fin-rot.html

Further Reading:
www.americanaquariumproducts.com/Aquarium_Medication.html

Carl

[devonjohnsgard]

I agree the treatment being provided now isn't going to help, so the disease will only progress. I would take Carl's advice to help stop the fot, which will be best for the little guy. Carl has helped a lot of fish successful heal. Best to you!