Aquarium salt is always something I have on hand. API General cure and Seachem MetroPlex/Kanaplex is also something else I would keep on hand.
Seachem Metroplex contains Metronidazole, which is an anti-biotic/ anti-protozoan medication.
API General Cure contains Metronidazole and Praziquantel. Praziquantel is a deworming medication that treats tapeworm.
Kanaplex is Kanamycin, an anti-biotic.
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You should not have anti-biotics sitting around because they break down over time. They break down faster in warm, humid or bright conditions.
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The only thing you want for fish first aid is salt, water conditioner/ dechlorinator, and a digital camera.
Most fish health issues are caused by poor water quality. If the fish look off colour, do a big water change and gravel clean the substrate every day until you find out what is wrong with the fish.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Salt treats lots of things ranging from minor bacterial and fungal infections, some external protozoan infections, gill flukes, anchorworm, microsporidian, etc.
The digital camera is to take pictures of the fish and post them on here so we can identify the problem.
The following link is pinned to the top of the emergency section of this forum and tells you what to do if your fish get sick. It's long and boring but worth a read when you have spare time.
If your fish ever looks sick or unwell, then the following steps might help. Test the Water and Clean the Tank. Test the water quality for ammonia, nitrite, nitrate and pH and write the results down in numbers. Check it for general hardness (GH) and carbonate hardness (KH) too if you can, but...
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