black mark on edge of fin?

[simmich]

Hi - first post here from relative newbie.

I've recently (9 days ago) moved my fish from a 29G tank to a 75G - water tests for ammonia/nitrates/nitrites are all fine as had existing filters, stones, plants moved over. New tank has an eheim pro 2 filter as well as 2 small HOBs from old tank.

However, have noticed a black mark on the edge of (baby) koi's pectoral fin - was initially a spot, now more of a line. However, fish is as active, shiny and demonstrating healthy appetite as always.

I've googled for fin rot and not sure if this is the case as she appears very active and healthy other than this mark and the fin does not appear to be fuzzy or deteriorating.

(I'm aware I need to find a pond for the koi soon - was a gift from a friend who believed the LFS when they said the fish would only live 3-6 months... I'm in Dubai, standards/knowledge here not always up to par.)

Would appreciate any suggestions or advice, particularly specific medicine/ingredients in case I need to hunt something down. There are several fish shops here but supplies are sometimes not in english or known US brands! Also, I may have to go out of town for 3 days - is this something I should be here to keep an eye on?

Thanks.

 

[lucy42083]

Hmmm black marks on the edge of fins is usually a sign of fin rot, which is due to poor water quality. What exactly are your parameters (I know you said 'fine', but that can mean different things to different people)? What temperature is the tank the koi is in? They are a cold water fish though they can tolerate a wide range of temps...I don't know much about koi though, so I don't know what that range is, or how it may affect them if the water is too warm. To be on the safe side, I'd do a few partial water changes to keep the water as clean as possible.

 

[simmich]

Thanks for your response.


Just tested the water and seems there's been a spike in nitrates. Ammonia and nitrites remain at 0. Doing a partial water change now and will monitor closely.

I suspect the problem originated in the old tank as water quality slipped the last few days they were in there due to my hectic work schedule and combo of small size/less efficient filters/messy fish. Not a mistake I want to happen again! New tank has better filtration and several plants which hopefully will help w nitrates as well.

How quickly should I add a medicine/treatment if it doesn't start to clear?

The other fish in the tank are 1 other baby koi, 2 blood parrots and 2 red tail loaches. All appear healthy and happy with exception of that one black edge on the koi fin... very worrying bc it's my favorite fish of the bunch!

 

[simmich]

PS - water temp is approx 70 F.

 

[jaylin]

Cleaning the water up should help a lot, try to keep the nitrates down below 10ppm for a while. (I understand with koi, even in a large tank, that can be a bit of a challenge sometimes.) If you see it worsening, you'll want to start medicating right away. Otherwise, you can give the cleaner water a few days to see if that helps. The only problem with fin rot is that it can go from just being noticable to omg, my fish just melted!! really quickly. So keep as close an eye on it as you can.

If you have to leave for a few days, I say feed very lightly for a few days prior to leaving, do substantial water change right before you go and don't have anyone feeding while you're gone. That will help keep the nitrates down while you're gone.

 

[jm1212]

you will definitely need to up water changes; when a fin starts to turn black rapidly, especially in koi and goldfish, you should be cycling in new water very frequently even if parameters are good.