The only thing you should have on hand for fish is salt (sodium chloride) and a dechlorinator. If you are really concerned, then have a bag of Activated Carbon or Highly Activated Carbon, and a bag of Ammogon/ Zeolite (ammonia adsorbing granules) too.
Most fish health issues are caused by poor water quality and a dirty environment. If you do big regular water changes and gravel clean the substrate, and clean the filter, the fish should remain healthy for years. In established aquariums, most diseases occur when new fish are introduced without being quarantined first.
The only reason you should have chemicals sitting around waiting to be used is if you have new fish coming in all the time. Then a number of them will require treatment, but for an established tank, there should be no need for medications.
The first thing to do if your fish get sick is test the water, wipe down the inside of the glass, do a 75% water change, gravel clean the substrate, clean the filter if it hasn't been done in the last 2 weeks, and increase aeration. Then depending on what is going on, you identify the problem and fix it.
The following link has information about what to do if your fish get sick. Some of you have read it and others might not have. Its worth a read when you have some spare time. It's long and boring so print it out and read it in bed to help fall asleep.
https://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/
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Using Fishmanic as an example. Most of your medications will break down over time and if exposed to sunlight or heat. This includes human medications, as well as anything used to treat birds, reptiles, fish or animals. The medications need to be kept cool and dry and will break down quickly when kept near an aquarium or heat source. I kept my medications and test kits in plastic icecream containers on the bottom shelf in the fridge (not in the vege crisper, just the bottom shelf).
*NB* Make sure children and animals can't get to them because most are poisonous.
Levamisole and
Praziquantel are used to treat intestinal worms. Theoretically, if you have treated all your tanks correctly and for the appropriate amount of time (once a week for 3-4 weeks), and at the same time, none of your fish should have intestinal worms unless you feed the fish live daphnia or other organisms from lakes or ponds containing waterbirds and fish. Then the live foods could carry intestinal worms back into the tank.
Intestinal worms don't kill fish quickly and any fish infested with worms, can easily last a few weeks while you get medication for them.
If you suspect the fish have intestinal worms, feed them more often (3-5 times per day) until they are treated, and for a month after they have been treated. The extra food will help the fish grow new blood, which is what the worms feed on.
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Mardel Quick Cure contains Malachite Green (aka: Victoria Green), which is used to treat external protozoan infections in fish. Common external protozoan infections include whitespot (Ichthyophthirius), velvet (Oodinium), Chilodonella, Costia and Trichodina.
*NB* Malachite Green is carcinogenic (causes cancer) so avoid coming in contact with it, and wash your hands with soapy water after handling the medication or working in the aquarium.
Whitespot and Velvet can be killed by raising the water temperature to 30C and keeping it there for 2 weeks. Chilodonella, Costia & Trichodina can be treated with salt (2 heaped tablespoons per 20 litres). Heat also kills them and a combination of 30C and salt will treat most external protozoan infections.
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Salt (sodium chloride) will treat minor fungal and bacterial infections, as well as most external protozoan infections. Salt will last for years and as long as it's kept dry, you can have a bag of it sitting around for 10 years and it won't go off.
More information on using salt can be found in post #2 at the link provided above.
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API Fungus Cure contains Victoria Green (aka Malachite Green) and Acriflavine.
Acriflavine kills fungus and some bacteria. It is reported to kill Columnaris (Mouth Fungus) but I have never used it for that so can't say if it's effective. And it kills external protozoan infections.
Acriflavine is not considered carcinogenic but is harmful if ingested, inhaled or you get it in your eyes. Wash hands with soapy water after using it or working in the tank.
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For Fishmanic, if you want to keep chemicals for the fish, your API Fungus Cure covers fungus and protozoan infections so you don't need the Mardel Quick Cure.
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I mention carbon and Ammogon at the start. If you suspect the fish have been poisoned and don't have the ability to do a huge water change to dilute the poison, then you can add carbon to the filter to remove the poisons.
If you have a filter break down or there is a sudden ammonia spike and you don't have the ability to do a huge water change to dilute the ammonia in the water, you can put some Amm
