Fish dying for no reason??

[Kiera17]

I have recently been having trouble with my tank. I have a 54 litre tropical tank which I keep a betta, harlequin rasbora and many cherry shrimp in there. But today, I have found 5 shrimp dead and every day I’m finding at least one shrimp dead. I’ve lost 4 harlequin rasboras as I noticed that one of them had mouth rot and a small amount of fin rot on its tail so I treated the tank for fin rot last week but lost 4 harlequins and lots of shrimp in the process. I have been testing my water parameters and everything is zero. The harlequins and shrimp are hanging around at the top of the tank so just in case it was due to low oxygen levels I’ve turned on an air pump in the tank. I feel as if my tank is cursed as before I kept the harlequins and betta, I had guppies and platys but they all mysteriously kept on dying whilst they looked perfectly healthy with no visual signs of illness whatsoever so I kind of gave up with live bearers and tried something new like the harlequins but I’m not having any luck does anyone have any ideas what this mysterious illness is that is killing all my fish and shrimp and what I could treat my tank with. I ideally do not want to take my whole tank apart and disinfect it as it’s been cycling for over a year so I don’t want to lose the beneficial bacteria. Would adding methylene blue be a good idea as I believe it’s an antiseptic treatment?
Thank you in advance for any help

 

[Byron]

We will need more data. Have you tested ammonia, nitrite and nitrate, and what are the numbers if you have? What is your GH (general or total hardeness, for the tap water is all we need)? And pH?

How often are water changes, and what volume of the tank? Which conditioner do you use?

Fish at the surface would suggest water problems; a significant water change might be in order.

Using treatments is always risky; many stress fish and invertebrates, and unless they are the best for the problem, will cause more harm than good. Others may be able to offer some suggestions with the data.

 

[Demeter32]

There is always a reason, the main being poor water quality. We need to know water parameters to be sure, but I think your water is bad. I'd be changing a good 50-75% ASAP. Don't dose any meds unless absolutely necessary.

How long have you had the tank set up?