Lifeguard by Tetra 

You may dose this in the fish tank but it would probably be better to improve the fish tank water quality, filtration and temperature WHILE treating the fish in a separate facility. 
But this can also be used as a several-hour bath. I like it for bathing fish when they can't be injected for logistical or size reasons. 

From Amazon.com
With Lifeguard Tablets, there is no need for time-consuming guesswork when it comes to treating your freshwater fish. A unique, broad spectrum, non-antibiotic agent, Lifeguard treats the clinical signs of disease at their earliest stages in freshwater fish. Added to aquarium water, the powerful oxidizing action of HaloShield attacks and destroys microorganisms that can cause disease in fish.2-Pack (32 tablets per pack)
Use for treating fungus, ick, red streaks, milky or shedding slime, flukes, bacterial gill disease, mouth and fin rot, clamped or torn fins, and ulcers. Lifeguard Tablets may also be used as a preventive when adding new fish. Five day treatment.Effectively treats most common fish diseases
Will treat ick, fungus, mouth rot, and more
May be used as preventative when adding new fish
For all freshwater aquariums

Feel nerdy? Want the "whole story" on Lifeguard and why it's so great?