Loach, Skinny Disease?

[Plecc]

Hi all, 
 
A month ago I sold off the majority of my bristlenose plec's. 
In the process of catching them the whole tank was stripped down, cleaned and redesigned. 
A day after selling the plec's I added another 3 zebra loach, to give me 6 in total and help keep up the bio-load of the pleco's I had just removed. 
A week or two later I noticed two of the old loaches were not looking so great. 
So I started dosing melafix at 50%, up to 75% over three days, by the end of that week they had both died.  
 
The last of the old loaches became very elusive after loosing her buddies. 
 
I just got back from work and found her at the front of the tank looking terrible. 
 
Her coloration is much darker than normal, her belly is slightly concave, barbell's are completely gone and her dorsal fin has either been nipped or has a bit of fin rot. 
She is also having issues with her balance. 
 
Now, a month on from the pleco sale, the three new loache's appear very active and healthy. 
 
Any Ideas what could be going on? 
 
All i can think is that, when I added the new loache's some sort of infection came with them that they had managed to build up an immunity to. 
 
 

 

[N0body Of The Goat]

It could well be the new loaches have infected the old ones, this is why we should all be quarantining new fish for a few weeks, to be more confident they are disease free and give them their first regular good sized meals in weeks or worse (fish passed through the supply chain get little food and fish stores often underfeed).
 
It could also be a major aggression breakout while the group have tried to rejig their normal loach hierachy, with the new ones wanting to be dominant over the old three. Conflicts will often show up initially through "graying," where conflicting fish lose their normal vibrant colouring.
 
At this stage, if the final old loach is still with us (fingers crossed), I would pull her out of there and quarantine her if the new three still look good. 10-20% daily water changes, if the fin damage looks like fin rot rather than fin nipping, a good anti-bacterial medication should be dosed (perhaps starting with half dose unless the meds specifically says it is safe with scaleless fish like loaches). 

 

[Plecc]

Thanks for the reply,
 
I'm sure it can't be an aggression issue, the three new loache's are young adolescents. So about 1/3rd the size of the old adults.
They are probably annoyingly inquisitive,
But appear to be submissive to the adults.
 
To be honest I'm 99% sure its not fin rot, her damaged dorsal fin has to be a nip but i think it is more likely to have been one of the catfish or giant danio's
 
I haven't managed to find her in the tank today, I'm not sure if this is a good or a bad sign.
 
I know your completely correct about the quarantine tank, but unfortunately it's being used at the moment as a nursery tank for some more plec's
 
In this situation, do you think it would have made a difference?
Wouldn't the new loache's still be carrying the bacteria after a month?
 
Whatever the problem is, it's been in the tank for a month now, so honestly i would be much happier treating the whole tank.