Greetings...I am a new player and poster. I have had the game for about a week or so now. I found that once I researched level 2 on environment, I would have certain breeds that would die of between the ages of 20 and 25 depending on the fish.

In order to sell them before they die, I would give them a dose of vitamins if there were ones near death at 19. Then you are able to stop the time in the options menu which will halt their health decline until they sell in the store. You can sell fish in the store while the time is stopped. You can also adjust the prices of the fish while the time is stopped. I did it by accident once and thought it might help with the ones that were dying shortly after maturity.

I have done this successfully many times and cut my death losses at that point in the game to almost nothing. Hope this helps!

fishtek ><{{{{{{•>

Hope this helps.

The declining health and subsequent death of some of your fish is due to insufficient environment. Some breeds of fish are rare enough that they only survive (and thrive) in high levels of environment. Researching the last level of environment will stop the problem you are encountering.