Sometimes it's the degree of what is in the medication.
Malachite Green is a powerful, though toxic medication with anti-protozoal and anti-microbial properties. A few cautionary remarks should always be associated with its use: To carefully measure the ACTUAL gallonage of the treatment system, as overdosing with Malachite is very easy to do... To take care not to spill or splash this material around as it is the dickens to get the bluish stains out of clothing, skin... or aquarium silicone. To use with caution period, as Malachite Green is toxic... poisoning your fish.
It is suspected of being a carcinogen as well as teratogen (a chemical that can alter DNA make-up), and respiratory poison. It is NOT to be used lightly. This material is also phytotoxic... poisonous to aquatic plants and terrestrial ones that may come in contact with it from splash, spray or your dumping aquarium water on them from your treated tank.
Quick Cure is another I would not touch.. Do not exceed recommended dosage, may stain silicone sealant and aquarium decorations. This product contains a chemical known to the State of California to cause cancer.
Maracyn I, Erythrmycin and Tetracycline are all the same thing- gram positive anti-biotic.
It would be better to get Maracyn I & II to use together rather then just Maracyn 1 and one of the others.
Triple Sulfa is the same as Maracyn II, gram neg antibiotic.
Furan 2 is a gram neg and pos medication, putting all those together in one- but it's a very powerful medication and caution is needed that you don't overdose, even the slightest. (You wouldn't want to with any medication as it is..)
Methylene Blue and Super Ick are the same things basically- ick medication.. you wouldn't need both.
The parasite tabs.. sometimes they work, sometimes they don't.. General Cure is also an anti-parasitic medication and does the same thing.
Basically, I would just get Maracyn I & II, General Cure... can treat ick with AQ salt and heat.