No, not a sickness, but in the box on the lower right, there's one bar that tells you the Health of the fish and one for how hungry it is. I know I can fix the bottom one by feeding the fish, but once the top Health bar starts to decline, there seems to be nothing I can do. This morning I had only two fish left - their health declines and they just die off at about age 50 or so. But I was able to keep one is 349 years old and its health has never waivered from 100%. I noticed the other, which was 35 or so, was starting to lose health. I bred the two, but when the babies got 5, they died and soon after the 35 year old died too. So I'm about to start over again for the 2nd time in a week. I just barely have enough money to buy eggs, but the last time I did that, I only got two or three fish who lived, might as well start from scratch again.