You should only use anti-biotics (Kanaplex) if the fish has a known bacterial infection that has not responded to normal fish medications. Misuse and over-use of anti-biotics can lead to drug resistant bacteria that can kill people, animals, birds, reptiles and fish.
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The white fluffy stuff on the caudal peduncle region is fungus that has infected some damaged tissue.
The black dorsal fin looks like chemical burns or bruising.
The emaciated appearance is probably intestinal worms and gill flukes.
The best treatment for all of these are clean water, lots of food (preferably frozen but defrosted foods or live foods like brineshrimp) and salt to treat the fungus and gill flukes. Paziquantel will treat the fish for gill flukes and tapeworm. Levamisole will treat the fish for thread/ round worms. You can use salt and Praziquantel at the same time.
You can add rock salt (often sold as aquarium salt), sea salt or swimming pool salt to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres of water. If there is no improvement after 48 hours you can double that dose rate so there is 2 heaped tablespoons of salt per 20 litres.
If you only have livebearers (guppies, platies, swordtails, mollies), goldfish or rainbowfish in the tank you can double that dose rate, so you would add 2 heaped tablespoons per 20 litres and if there is no improvement after 48 hours, then increase it so there is a total of 4 heaped tablespoons of salt per 20 litres.
Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.
The salt will not affect the beneficial filter bacteria but the higher dose rate will affect some plants. The lower dose rate will not affect plants.
After you use salt and the fish have recovered, you do a 10% water change each day for a week. Then do a 20% water change each day for a week. Then you can do bigger water changes after that.
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You can use Praziquantel to treat tapeworm and gill flukes. And Levamisole to treat thread/ round worms.
Remove carbon from filters before treatment and increase aeration/ surface turbulence to maximise oxygen levels in the water.
You treat the fish once a week for 3-4 weeks. The first treatment will kill any worms in the fish. The second and third treatments kill any baby worms that hatch from eggs inside the fish's digestive tract.
You do a 75% water change and complete gravel clean 24-48 hours after treatment.
Treat every fish tank in the house at the same time.
Do not use the 2 medications together. If you want to treat both medications in a short space of time, use Praziquantel on day one. Do a 75% water change and gravel clean the substrate on day 2 & 3. Treat the tank with Levamisole on day 4 and do a 75% water change and gravel clean on day 5, 6 & 7 and then start with Praziquantel again on day 8.
