Help with white growth (not ich) on cory fins

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May 21, 2020
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Hi all

I have been keeping several cory spp. for about three months. Some of the fish have a white growth on their fins and not on any other body part (please see pic). The growth is rather large white usually round but sometimes goes along the fin appearing on the caudal or dorsal fins mostly. Some of the fins are damaged too (secondary fin rot?)

I see no clear signs of difference in behavior. I should mention that the pH is around 8.2! but the water is soft (see below) I was able to breed some of the spp. sporadically. Also, I feed bloodworms almost daily (together with dry feed and some artemia) and I cycled the tanks with some of the fish. The phenomenon is spreading slowly to other fish and other tanks and so far is non lethal. Any idea what this is? Would appreciate your advice on identification and treatment.Please see all information below:

Species and status:

Aeneus bronze and albino - infected

Venezuelanus- infected

Simulatus- infected

Panda- not infected

Septentrionalis- not infected

Paleatus- not infected

Water parameters:

Temp range: 23-26 deg C

pH: 8.2!!

Gh: 6 Deg. German

KH: 5 Deg. German

Ammonia, nitrite are 0 and nitrate is low around 10 ppm, 40% - 50% water changes were made very frequently at first daily for the first week and the every two days in the 2nd and 3rd now once a week. No chemicals were used.

Tanks are 200 liters some are divided by a screen into 100 l tanks, no substrate, sponge filter and air stone, no heater (temp is regulated with AC), java moss in some of the tanks. Tanks are starting to get brown algae in them as happens after cycling. Tanks have been running with fish for about three months. I have moved and added a lot of fish around about a month ago but have seen this on a few fish before the big change was made (I had only bronze and albino at the time)

I recently tried to treat with directly applying methylene blue to the infected areas, possibly there is some if not major improvement.

Any advice will be greatly appreciated.

Thank you


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Hi Colin, I must have missed something, What was your answer? Thanks
It's excess mucous, fungus and fin rot.

This is normally caused by poor water quality, however your water looks good in that respect. It can also be caused by chemicals in the water.

Are you adding anything to the tanks (fertiliser, pH buffers, anything)?

Do they have anything on the bottom of the tank (sand, gravel) or is it just a glass bottom?

Wipe the inside of the glass down with a clean fish sponge.

Clean the filters if they haven't been done in the last 2 weeks. Wash filter media/ materials in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn.

Do a 75% water change and gravel clean the substrate if you have any.

Add 2 heaped tablespoons of rock salt, sea salt, or swimming pool salt for every 20 litres (5 gallons) of tank water. Keep the salt in there for 2 weeks.

If you do a water change while using salt, add salt to the new water before adding it to the tank so the salinity in the aquarium remains stable.

The salt will not affect the filter bacteria, shrimp, snails or plants.

If there's no improvement after a week with salt, post more pictures and we can talk about chemical medications.
Hi Colin
Thank's for your quick reply. I did not add anything to the tanks and have no substrate. Will follow your advice (probably should have cleaned the filters a while ago..). Regarding salt, I have used salt in past with other fish and this is always the first thing I do with sick fish but I read that corys are very sensitive to salinity changes so just double checking. Thanks again!
That level of salt (2 heaped tablespoons per 20 litres) is safe for all fish, snails, shrimp, filter bacteria and plants.
The lack of a substrate is likely behind this. Colin's advice on wiping down the ghlass including the tank floor is why. Bacteria wil live on that surface and cories are susceptible, whereas a sand substrate avoids this issue. Over on Corydoras World, Ian Fuller has posted that bare tank floors need this thorough cleaning every day, but a substrate avoids this, plus the cories expect a sand substrate because they are filter feeders so this is likely stressing them as well.
Thanks Byron. I got the same answer from another forum so I am definitely getting some substrate. The sand in our area is very salty so would need to get it from another source. I got a recommendation to get silica/quartz sand used for pool filters. Is this good? Thank you!
Thanks Byron. I got the same answer from another forum so I am definitely getting some substrate. The sand in our area is very salty so would need to get it from another source. I got a recommendation to get silica/quartz sand used for pool filters. Is this good? Thank you!

No, too rough and angular. Either buy aquarium river sand, or you can use some brands of play sand. We have been discussing this in another thread today, this was the sand:

I use Quikrete Play Sand but it may only be available in NA, not sure. There is also I believe Argos Play Sand in the UK.
I see, thank you and thanks to all who replied. Got some great information here.
I've read that this is a fat deposit and is typical in Corydoras. Varying the diet can help, but they usually go away on their own. Taking all the above steps is also good as well.

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