When it comes to "natural" glassfish, I am not sure. The painted variety seen in a lot of pet stores do usually die within a short time. They are given shots of dye to make them colorful. This dye is to the fish like a cold or flu is to you. Their body tries to fight it off which is what your body does when you have a fever. This makes the fish weak. Remember how you feel when you are sick? They can't get rid of the dye because their body can't fight it off so they remain weak. You are lucky in that your body can fight off colds and the flu. This is why most of them don't live for very long. A few are stronger and do adapt to the dyes used, but most of them are very weak. I would not buy a dyed fish. This is very cruel to do this to a fish. The regular kinds of glassfish which you can see their organs and bones through are probably stronger fish. I have not bought any of them so I can't be sure.