I agree with skjl47, all medications (other than plain salt) are toxins, lightly dosed to kill microbes, think chemotherapy.
Those toxins can kill fish if not dosed properly, or mixed randomly
Unless you know what you're treating, how can you just add stuff.
A few questions
How long has your tank been set up?
Was it properly cycled before adding fish (properly cycled does not mean just sitting a few days)
I see it looks bare, unless your filter his a ton of bio-media this can be a a problem.
Most fish ailments start because of stress, if your tank is bare (no cover for the fish to feel comfortable in) this is stressful.
As mentioned above, to keep fish like FHs, you need to know the fluctuations in composition of your water, ammonia, hardness, pH, a few others. These water parameter factors let you know when water changes are needed, and if something is awry (especially for new fish keepers).
The proper keeping of fish is more like a science chemistry, and biology experiment, rather than a simple hobby like stamp keeping, or even bonsai trees. (its more like trying to keep orchids alive)