dying harlequin tetras

[fishwarder]

A while ago one of my harlequin tetras died. It was swimming upside down and seemed to have an injured fin.. I figured it had just hurt itself or been bitten and was a one off. But now all my other harlequin tetras seem to be turning white, and another one died. Now a third is starting to hang around at the bottom of the tank like it's having trouble swimming (the fins seem kind of stiff.. they start to gasp a bit..). I don't know what's wrong, or what to do about it. I would rather no more died but I especially am hoping this is something that won't spread to my other fish (bloodfin tetras, guppies, rosey golden barbs, corydoras). Please if anyone has advice.. fire away

[juggler]

Hi! Welcome to AQ!

Concerning your question, there could be several reasons. So need yourself to sort out.

Are the harlequins new additions to the tank?
Any problem with other existing fish?
Anything you did that could have altered the water chemistry -- like major water change suddenly, washing the filter, adding chemicals/medications?
How about the temperature?
Notice anything unusual floating on the water surface?

In my case, there was once where my entire harlequin population (~20) was wiped out in my mature tank, leaving other fish intact. I could only think of the heat during that period and a sudden occurance of a dust-like floating algae in that tank. I used a surface skimmer to remove the floating stuff. But I did not replace the harlequin population after that to test the water. So I can't say for sure if my observations are correct.