Diagnosis? TW photo of dying guppy

[Elena82x]

Hello
Can anyone visually identify what is wrong with this guppy? I’ve had 5 guppies die now over a number of weeks, no apparent symptoms. The others died suddenly (lethargy over 24-48hrs then dead) though this one has been at the surface for a couple of weeks more and more (plenty of aeration and the other fish aren’t at the surface) until clearly dying today so I have just euthanised :’-(
It is a fairly new tank (3 months old tank 3 months in this tank) and I’ve had this guppy 3 months and didn’t quarantine (didn’t really know about quarantine until recently).
Water conditions are kept well (no ammonia nitrites, nitrates kept around 10) PH 7.4. I had tried salt for 3 weeks though low dose due to having cories. I was about to try a higher strength salt bath and had just prepped it when I found him struggling more.
The only other fish with an issue as I can tell is a molly who has been flashing for weeks and I can’t work out why. No others flash at all.
I just thought I ought to try posting a photo in case there is anything obvious that you might see that I haven’t. - if you’re wandering what the oily water is it is the clove oil I was starting to add
Thank you.

 

[connorlindeman]

Guppies nowadays are not the hardy fish they used to be. Many times when you buy them at the store, they are already sick.
The tail on that guppy seems to have fin rot. This is caused by poor water quality.
How often do you do water changes?
Did you cycle the tank before adding fish?

 

[Elena82x]

Guppies nowadays are not the hardy fish they used to be. Many times when you buy them at the store, they are already sick.
The tail on that guppy seems to have fin rot. This is caused by poor water quality.
How often do you do water changes?
Did you cycle the tank before adding fish?

It was like that when I got him so maybe he’s been ill since I’ve had him the tank cycled before his arrival and I do water changes usually 40% twice a week and test pretty much daily. Thank you for your reply.