Harlequin Rasbora With Missing Fin

[mrabz]

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Tank size: 40g
pH: 7.0
ammonia: 0
nitrite: 0.1
nitrate: 5
kH: unknown
gH: unknown
tank temp: 24C

Fish Symptoms (include full description including lesion, color, location, fish behavior):
Noticed this morning that one of my harlequins is missing it's Dorsal Fin. Has a pinky / red cut or lesion where the base of the fin should be. All other Harlequins + other fish are acting as usual, this Harlequin has become secluded.

Volume and Frequency of water changes:
40g and 20-40% twice weekly

Chemical Additives or Media in your tank:
Nutrafin Aqua Plus
Easy Life Profito
Easy Life Easy Carbo

Tank inhabitants:
25x Guppies (approx)
4x SAE's
1x Bala Shark
6x Harlequin Rasbora (including the afflicted fish)

Recent additions to your tank (living or decoration):
The harlequins are the newest addition around 2 weeks ago

Exposure to chemicals:
As above.

Digital photo (include if possible):

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EDIT: Just noticed two other males(?) doing a "dance" and read up further on a google search. Could this be the result of one during the night or is something else going on?

 

[fluttermoth]

I don't think it is 'missing' it's dorsal fin; it's just got it clamped down for some reason; possibly feels ill because of the raised nitrite? Or is just being submissive to the other males?

You need to do a big water change to get rid of that nitrite anyway, so that should be your starting point. Do you not have a background on your tank at all? That could make your fish feel quite insecure.

I feel I ought to point out that the bala shark is not suitable for a 180l tank; they need much more room than that and really ought to be in a group.

 

[mrabz]

Thanks for your reply, the nitrate is low (according to my testkit) plus it is due a water change tomorrow.

The tank is in an alcove in the living room and has plants growing at the rear, they aren't tall enough yet to cover the back!

Regarding the bala shark, there were two sqaushed into a 35 litre I liberated from a 20 year old trash girl with angels, gouramis and a betta one has gone back to a LFS I kept the other one.

Will do a water change when I get home from work.

 

[hensonc4098]

I'd agree with fluttermoth that it's got it's fin more clamped than missing. The "dances" you've seen will either be mating rituals or dominance behaviour to establish who is the boss. They won't hurt each other (or not that I've seen in keeping them for several years) but will just show off to each other to figure out who's better.
The clamped fin could be due to this being a subordinate fish that doesn't want to get involved in the hierarchy, or could be due to that nitrite. I would have suspected more fish would be affected if the nitrite was to blame though.
How long has the tank in general been set up? Seems a bit odd to have a nitrite reading 2 weeks after addition of just a few fish...

 

[mrabz]

Cheers hensonc4098, I did some reading up while 'working' today. Tank's been set up just over a month, and used filter media, substrate and some water from an established tank.

The Harlequins are the newest addition to the tank (2 weeks ago). See my sig for tankmates.

Done a water change, the no3 is still ~5mg/L but slightly clearer compared to the 1st colour on the chart. He's still swimming around with his clamped fin, but there's no more rituals from what I can see now.

 

[Alm0stAwesome]

That fish is a "she" and it's possible that the fin has somehow been damaged at some point as, maybe it's just me but in the picture it looks a little white? Make sure your water is very clean - however I tend to start thinking about adding treatments when I see fin tears looking white maybe someone else will chime in on that one