LOL, you know me too well
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Bacterial infections are usually red and cause the tissue to rot away. This is not a bacterial infection.
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Furan 2 and Kanaplex are both anti-biotics that should only be used on known bacterial infections that have not responded to normal fish treatments. Improper use and mis-use of anti-biotics has lead to drug resistant bacteria that kill birds, animals, reptiles, people and fish.
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Now I'm gonna throw my 2 cents worth in.
How long have you had the fish for?
How long have the fish had the creamy white patches?
Have you added anything to the tank in the 2 weeks before this started?
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The fish have creamy white patches of excess mucous over their body and face. This is caused by something in the water irritating the fish, and in turn the fish produce more mucous to protect themselves.
Fish naturally have a thin layer of clear mucous over their body and fins to protect them from poor water quality and disease organisms in the water, and to help them move through water easier. If they are stressed out by something in the water, they produce more mucous and this excess mucous appears cream, white or grey, depending on how much is there.
If the excess mucous covered the entire fish from nose to tail, and all the fins in between, I would say it is water quality or chemicals irritating them. However, when there are cream, white or grey patches over their body, it is usually caused by external protozoans like Costia, Chilodonella and Trichodina.
The best treatment for these types of external protozoan infections is salt, Malachite Green or copper. Salt being the safest, yes, even for Corydoras, (see directions below for using salt).
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Before you add salt, do the following:
Work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
If you have big rocks or driftwood in the tank, remove these so you get a more accurate water volume.
When you measure the height, measure from the top of the substrate to the top of the water level.
You can use a permanent marker to draw a line on the tank at the water level and put down how many litres are in the tank at that level.
There is a calculator/ converter in the "FishForum.net Calculator" under "Useful Links" at the bottom of this page that will let you convert litres to gallons if you need it.
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Wipe the inside of the glass down with a clean fish sponge.
Do a 75% water change and gravel clean the substrate. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn.
Increase surface turbulence/ aeration when using salt or medications because they reduce the dissolved oxygen in the water.
Stop adding chemicals/ fertiliser, etc, for a few days and see if the problem clears up with just clean water. If they still have the cream white patches 24 hours after a big water change and no chemicals added, then add salt.
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SALT
You can add rock salt (often sold as aquarium 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 (4 heaped tablespoons per 20 litres) will affect some plants and some snails. The lower dose rate (1 or 2 heaped tablespoons per 20 litres) will not affect fish, plants, shrimp or snails.
After you use salt and the fish have recovered, you do a 10% water change each day for a week using only fresh water that has been dechlorinated. Then do a 20% water change each day for a week. Then you can do bigger water changes after that. This dilutes the salt out of the tank slowly so it doesn't harm the fish.
If you do water changes while using salt, you need to treat the new water with salt before adding it to the tank. This will keep the salt level stable in the tank and minimise stress on the fish.