Second Sick/dead Fish In A Month

[NJPeggySue]

I wouldn't say I'm new to the hobby, but I  didn't know where else to post this.  So I've had my 20-gallon tank running continuously for approximately 10 years.  In those ten years, I've never had a problem with disease.  However, in the last month the weirdest thing has been happening.  Two of my fish, both tetras but different kinds have had the same symptoms. The second is in the process of dying right now. I have no idea what they have/had but I don't want my other fist to get it.  Anyway, they both were laying on their sides spinning in circles.  Like frantically spinning in circles, not lazily or floating in circles but spastically swimming in circles. Basically, like they had no control over their bodies. I tried treating them for swim bladder issues, with medicine, salt baths, and peas obviously this had no effect on them.  I of course, googled it and whirling disease came up.  Essentially what I want to know is if whirling disease can even exist in a tropical tank?  I read that it can't but I don't know what else they could have.  I removed them both from the tank when I realized they were ill and hoped that this would prevent other fish from catching whatever they have but now the second one is sick and the first fish has been out of the tank for approximately two weeks.  Anway, I feel like this email is very scattered but any assistance would greatly be appreciated I don't want to lose any more fish.  The one that is sick now, I've had for five years.  I've never had problems before and don't know what could be wrong now I"m going to test the water now, but wanted to post this first.  I'm super confused about what's going on. Thanks for any help/advice you can give me. 

 

[Byron]

I would consider these as isolated cases of internal issues, and most are not treatable.  I have had fish that have been with me for many years suddenly develop the inability to "swim" and provided it is only the one fish I usually net it out and euthanize it.  Sometimes their ability to escape a net is remarkable, so I just leave them until I am able to net them or they die.  No point in stressing out all the fish trying to catch one.
 
I've no idea as to the cause, but it can be physiological or brought on by something.  Isolated cases are nothing to worry over.  And your two fish in a month or so are still isolated.
 
It is generally best not to treat an aquarium for any "possible" issue unless it is clearly probable.  Any treatments involving chemicals, salt, medicines, etc can cause more issues with these and the other fish, and that risk is not worth it.
 
Water tests are a good initial step, just to confirm.
 
Byron.