Long black spots on bottoms of mollies tails

[kreesdqban]

When I finally acclimated my new mollies and let them swim around in the tank I noticed that the 2 males had long black marks on the bottoms of their tales. The fish don't seem to be showing any other signs of illness or anything out of the ordinary. I saw on the illness thread that there is a black spot disease caused by some bacteria. I just wanted to clear things up, so I provided the best picture I could get.

 

[KarlTh]

Those are lyretail swordtails, not mollies, and that colouration is classic for red swordtails.

 

[kreesdqban]

Really? How can you tell exactly? Don't swordtails usually just have the long lower fin?

I was looking on google, and it seems that mollies usually have a straight forward facing mouth rather than facing up... is this right?

 

[KarlTh]

Lyretail swordtails have, by definition, lyreshaped tails. Mollies and platys/swordtails can look pretty similar, but I'd say that this

looks pretty like your fish.

It's really hard to say exactly why they look like swordtails rather than mollies, but they do. You may be on to something with the mouths; mollies' lower lips are below their upper lips; on swordtails they're at the same level, making the mouth more upturned.

 

[kreesdqban]

Yeah you're right. After looking closely at some mollies and swords it does seem like my fish look more like the swords after all. I was told they were lyretail mollies at the farm and thats what I went by.

Thanks for your help! So you say theres nothing to worry about with the black spots?

 

[KarlTh]

Nope. Many red swordtails have a black lower edge to the fin extension.