cory disease.. help!!

Question
hi there, i have 2 adult albino corys, who've laid several eggs in the past, out of the batches over the years i have kept 10 of the babies which are now2 years old, my problem is that 2 of them have died unexpectadley, with no symptoms whatsover.. except that to me they look like they have a dark mass inside them, when i removed the dead from the tank they smelled rotten. i also had a bronze cory who i think may have dies from the same thing, only with it being a darker fish i noticed that its colour was very pale and pachy looking, and that wehn i inspected it after it died it had a huge hole in the 'belly' area. Now 2 more of the small albinos have dark masses showing inside them and also  seems to have a split in its belly... i cant find anything to help me diagnose and treat them.. and although there are no really obvious symtoms, for the last 2-3 days they seem to stop swimming, altough they seem to try, lie on their side and there breathing becomes rapid, its a horrible death and one i would really like to prevent.

Answer
Denise,

 How are your water perimeters? Ammonia and nitrites need to be at zero and nitrates no more that 20ppm. The Ph has to be between 6-8. Rapid breathing is a sign of low oxygen. I think you have a bad water quality problem. Are you testing the water weekly and doing weekly water changes of 25%? As for the dark mass, with albinos that is very common. I have 2 while Betta females and I can see that in them. I am thinking the dead fish with the hole in its belly got it from another fish trying to eat it. They go for the soft places first. I would check the filter to make sure it is working properly and add an air stone to the tank. Do a 50% water change now. Don't forget to add a water conditioner and that the new water going in is the same temp as the tank. Tomorrow test the water again. If you have any readings of ammonia and/or nitrites, then do a 25% water change. Check the water daily and continue the daily water changes until both are at zero. What do you have in the tank with the fish? Plants, heater, filter, other decorations? Sometimes a decoration will have a sharp edge or point. When the fish swim by it will scrape itself therefore leaving a wound. I am still thinking it has to do with the water. If after the water changes you do not see a difference or they get worse let me know, but I think that once the water is brought under control things will look better for the fish. Please, let me know how things are going.