OK, I Googled it and this is what I found:
"While bubble eyes are as inexpensive and easy to find as other fancy varieties, they are not recommended for beginning fish keepers because their delicate eyesacs can easily be damaged by rough or sharp surfaces, or by the filter intakes. A bubble eye should never be kept in a tank with anything abrasive, such as most rocks. Should the eyesac become punctured, it is important to keep the water especially clean, or use a topical product like Melafix to stop the puncture from becoming infected. A punctured sac will eventually heal and grow back although it is almost always scarred or misshapen after healing."
Hope this helps!