What Is This?

[JenniG227]

Tank size: 20 gal
pH: 6.8
ammonia: 0
nitrite: 0
nitrate: 0
kH:
gH:
tank temp: 74

Fish Symptoms (include full description including lesion, color, location, fish behavior): 2 males guppies tails have disintegrated.  This happened very quickly.  This morning went in to check the tank and everything seemed fine, this afternoon after returning from work found the 2 male guppies tails almost gone and them staying at the top of the tank struggling to swim.  I have no idea what is going on.  None of the other fish in the tank seem to have any issues including a 3rd male that was introduced with these 2.   
Volume and Frequency of water changes: 25% every 2 weeks 50% every 4 - 6 weeks. 

Chemical Additives or Media in your tank: melafix with intro of new fish

Tank inhabitants: mollies, guppies, sword tails, tetras, danyo, 

Recent additions to your tank (living or decoration): 3 male guppies (2 of whom are the ones with the tail issues) were introduced to the tank 3 days ago. 

Exposure to chemicals:

 

[NickAu]

Volume and Frequency of water changes: 25% every 2 weeks 50%

I would be doing 25 to 50% weekly.
 
 
What are you using to treat the water when you change it? Eg Prime or EasyLife?
 
This is just my opinion.
 

Chemical Additives or Media in your tank: melafix with intro of new fish

Anything that ends in " FIX " is junk in my book.
 

pH: 6.8

I doubt this is  suitable  for live bearers like guppies and mollies.

 

[guppiegirlie]

I agree with NickAU you should be doing a 25-50% water change weekly and using a water conditioner to make sure the water is safe.

How many of each species of fish do you have? Female and also male guppies can be aggressive if numbers are out, IME 3/4 females per male is a good number.

Also what species of tetra do you have? They can also be quite nippy.

And again agreeing with NickAU adding anything to your tank when adding fish is fairly pointless, it doesn't really make any difference.

It could also maybe be finrot. With these being new they may well have been carrying the disease from the pet store. I had a batch of females not too long ago with the same problem, I would highly recommend esha2000 after a three day course repeated a week later all signs cleared up and my fish are all doing just fine again, fins and tail growing back nicely.

 

[JenniG227]

I have 2 swordtails, 4 gold panda mollies (3 of which are babies about 2 weeks old), 2 trigonostigma harlequin rasboras, 1 long finned pearl danio, 1 cardinal tetra, 2 female guppies, 1 male guppy, 1 spotted cory catfish.  
 
I use Seachem products: Prime, stability, purigen
 
It just happened so quickly I didn't know what was going on.  In the morning they were fine, eating, swimming etc.  That evening when I came home both looked like above pictures.  

 

[guppiegirlie]

I had this happen once over to two of my male guppies, the morning they were fine, swimming about happy and healthy looking. When I got home that night both had shredded fins and tails, one was dead the other almost dead, I had to euthanize him and still to this day don't know what caused it as all my other fish were perfectly fine.

But on another note you need more fish!

Tetras especially cardinals are schoaling fish and are best kept in groups if at least 6, I'd say the same for the rasboras and danios.

Also on advice I was given on here about Cory's (I've just changed substrate as I would like to add some in the near future) these are also schoaling fish and should be kept in groups of 8.

But back to your problem the only thing I would suggest is getting some esha2000 and treating your tank, as if it is fin rot it will spread.

Hopefully your little guys will bear better then mine and pull through, good luck

 

[eaglesaquarium]

I had this happen once over to two of my male guppies, the morning they were fine, swimming about happy and healthy looking. When I got home that night both had shredded fins and tails, one was dead the other almost dead, I had to euthanize him and still to this day don't know what caused it as all my other fish were perfectly fine.

But on another note you need more fish!

 
Well, more fish would be accurate for a larger tank, but with a 20 gallon, the limit has been met already.  
 
It would be best to move the mollies and swordtails to a new home, as neither is truly suitable to this water, nor to the size of the tank.  Both are really best in 30+ gallons.  The same is true of the danio as well, which while a small fish is a highly active swimmer and needs the length to really go.
 
Upping the rasboras, cories and cardinal numbers would be good after that though.

 

[guppiegirlie]

↑ exactly that.

I don't have much experience with mollies or swordtails but do know they can grow quite big, my dad has a pair of mollies and they're quite big! I didn't think they'd be suited to a tank that size.

 

[betta fish]

It is possible that the fish were attacked by another fish and would suspect the same with guppiegirlie too, for the fish to be fine and then in the space of a day shredded and looking unwell, I have found guppies especially males are quite cumbersome with those tails.