There is a saying that I really like: "Calm seas do not make good sailors." Which means we don't learn without problems. I have had over the course of keeping fish several diseases that I have had to treat, ick, and dropsy to name two. But what you are describing what is killing your fish I have yet to encounter, so I looked up fish diseases and found something that doesn't sound promising for you, if this is the problem.
Here is a quote from this website on fish diseases:
10 common aquarium fish diseases- How to treat your aquarium fish.
Now I don't mean to scare you with this because I really don't know what your fish have, it just sounds like this from your discription of several different problems seen in your fish.
Quote: "
Tuberculosis
The aquarium fish disease is caused by bacteria, such as Mycobacterium and Nocardia. The fish looks drained, often afflicted by secondary bacterial and fungal infections, the eyes are bulging and ulcers are formed on the body and internal organs. It is believed that the infection of fish occurs through the feed, although other methods are possible. Therefore, one should be especially careful when using fodder fish, since they are often kept in very poor conditions.
The treatment is extremely difficult and requires expensive drugs (containing sulfafurazole, minocycline, etc., so it is best to kill the diseased fish painlessly, if after quarantining to the quarantine aquarium, its condition has not improved. To prevent the disease, acquire new fish only from a healthy aquarium and quarantine them.
A number of bacteria from the Mycobacterium genus can be transmitted to people, causing various skin diseases, etc. Therefore, avoid contact with unprotected skin, especially with open scratches, etc., use gloves. After an outbreak of disease, always disassemble the equipment.
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