Harlequin Rasbora Has Dark Streak On Its Bottom

[xoedusk]

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Tank size: 10 U.S. gallons
pH: 7.0 - 7.2
ammonia: 0
nitrite: 0
nitrate: 20 - 40
tank temp: 75 deg F

Fish Symptoms (include full description including lesion, color, location, fish behavior):
This little guy is pretty small, maybe half the size of an adult. Today I noticed a small dark red or black patch on his bottom side, starting a few mm in front of this triangle and extending down about 1 cm toward his tail. This region might be a little bloated.

Volume and Frequency of water changes:
Weekly 40% water changes and gravel vaccing.

Chemical Additives or Media in your tank:
Only a dechlorinator*.

Tank inhabitants:
3 Platies
3 Harlequin Rasboras, inclusive.

Recent additions to your tank (living or decoration):
This guy IS a new addition. About 1 week ago. He has started to get picked on by his bigger rasbora mates it seems.

Exposure to chemicals:
n/a

I will try to post a pic later tonight when I am at home.

EDIT: here is a picture of him
http/students.washington.edu/bsteph/littleguy.jpg
A bit hard to see, but notice how his tummy sticks out a bit. Also, the mark has changed shape. It is not as long and is now touching his the base of his triangle.

 

[WILDER]

Is it black or red, it sounds bacterial can you issolate him.
If not you you will need a bacterial med for the tank.
Try some shelled pead for the bloated tummy he dosnt sound in good shape to be honest, sounds like a bacteria infection which is progressing fast.

 

[xoedusk]

We've moved him to a small isolation tank.
Here is a short and small video clip of the little guy swimming. He seems to be bobbing up when he stops swimming.
http/students.washington.edu/bsteph/littleguy.3gp
You may need to use Quicktime to view the video. Tell me if it won't play please.

EDIT: We've tried to give him some Tetracyline*, but he won't eat it.

 

[WILDER]

The video dosn't work.
Add a bacterial med to the tank if he wont eat the flakes.
As he stopped eating as that not a good sign.
Good luck.