If you can safely does so gently rub your finger across one of the blisters (one of the smaller ones) if it feels like it is filled with fluid or it pops, releasing a transparent to translucent yelowish fluid and making a slight crackling noise it may be a symptom of swin bladder disease. This is one of the less common symptoms of that disease. It can be treated by upping the areation level and dosing the tank with eithe erythromycin or quinine sulfate.
If the blisters multiply and follow the lateral line, especially if the fish also starts to show scales with raised edges, swelling of the abdomen, or pop-eyes then it is probably bacterial dropsy and can be treated by raising the temperature of the tank gradually to between 84 and 86 degrees farenhiet while dosing the tank with either kanamycin or doxycycline.