Crimson Spotted Rainbow With Pink Lesion

[Gruntle]

Hi, all was well this morning but when I got home a few minutes ago I found this:



Tank size: 200litre, 48" by 15" by 18"
tank temp: 27 degrees Celsius
I'm going to do a full water test now and post the results!

Fish Symptoms (include full description including lesion, color, location, fish behavior): pink lesion about the size of a grain of raw sugar on the gill cover. Fish seems fine as do all the others in the tank.

Volume and Frequency of water changes: 25% weekly! hasn't been done for just under a week.

Chemical Additives or Media in your tank: none

Tank inhabitants: 3x Rainbows, 6x beacon tetra, 6x cherry barb, 2x BN Pleco, 3x mystery snails

Recent additions to your tank (living or decoration): 3x cherry barb, 3x mystery snail

Exposure to chemicals: none

Digital photo (include if possible): see above.

 

[Gruntle]

Ph 6.6
Gh 120
Kh 0
Nitrate 40
Nitrite 0
Ammonia 0

Gh and Kh came from a test strip, sorry it's all I have to hand.

 

[Gruntle]

I've treated with Aquari-Cycline (Australia's answer to Tetracycline) and salt. Now we wait...

 

[Gruntle]

So after a week there's been no change to the lesion, so MTS has kicked in and I'm trying to get a hospital tank (the Good Lady Gruntle said it's fine, at least I'm not breeding racehorses).  A smaller tank should mean less medicine required (so obviously cheaper).  Wil keep you posetd. Although the fish appears to be happy, I don't like seeing it with a growth.
 
If all goes well I'm hoping to use the small tank to have a go at breeding Cherry Barbs later.

 

[Gruntle]

Bumping this.
 
The original fish with the lesion survived for a couple more months, then suddenly went very thin and passed away.
 
Now 3 of my 5 remaining rainbows have the same lesions. A fourth is very thin (noticeably big head which means the body must have shrunk) but no lesions. The fifth seems fine.
 
All are feeding well. All other tank occupants are happy. I don't know if it's something species-specific, if there's a water parameter issue (perhaps the rainbows are far more sensitive to something) or if there's another cause that I just can't get my head around.
 
I'll try and get some pictures tonight.
 
Any help would be appreciated.  I love my little natives and am sad that they are ill.