Any chance of a picture of the fish?
What symptoms are the fish showing?
What is the ammonia, nitrite, nitrate and pH of the tank water?
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How long were the fish in the shop tanks for?
Rummynose are sensitive to changes in water chemistry and suffer a lot of stress during transport and shipping. If the fish had only been at the shop for a couple of days when you got them, they would have been stressed from the move to the shop. Then they suffered more stress being caught, bagged up and moved to your house. Then more stress from the anti-biotics. And finally more stress being moved 3 days later.
If you are going to buy fish, try to get them after they have been at the shop for at least 1 week. Do not buy fish if they have only just come in, or if the shop has done a water change in the last couple of days.
When you get fish home, put them into quarantine and leave them there for at least 2 weeks to settle down and recover from the stress.
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If you are going to quarantine fish, you keep them in a separate tank for at least 2 (preferably 4) weeks and wait to see if any diseases appear. If no diseases appear after 4 weeks, you add the fish to the display tank.
If a disease appears, you treat it with the correct medication.
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Metroplex contains Metronidazole, which is an anti-biotic designed for people. It should only be used as a last resort on known bacterial or protozoan infections that have not responded to normal treatments.
Improper use and mis-use of anti-biotics has caused drug resistant bacteria that kill people, birds, animals, reptiles and fish.
Metronidazole is only used to treat fish for internal protozoan infections.
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For Guppy10, salt is fine for all freshwater fish as long as you don't overdose it. When treating Corydoras and tetras, you can use 1-2 heaped tablespoons of salt for every 20 litres (5 gallons) of tank water.