My angel fish keep dying

Question
I have an ongoing problem with freshwater angel fish. I will buy them and about a month later they die. They are fine, swimming normally and eating great. Suddenly, (and I mean one minute they are fine and 30 minutes to an hour later they aren't) they are listless and weak. Their swimming becomes erratic and they can't keep themselves upright. They go upside down, right side up, on their side, up, down or just get caught it the current. They are usually dead within a couple of hours. From the time I notice there is a problem until they are dead is just a matter of a few hours.

There is no sign of injury
There is no sign of a parasite. Their gills look normal and their breathing is normal. They do not gasp at the surface.
There is no sign of a fungus. They look completely normal.
With the larger angel fish I have lost, the only thing I have noticed is abdominal bloating. This starts about a week before they die.

I have started adding crushed frozen peas to their diet about 2x a week. Otherwise they eat tropical flake food about twice a day...sometime 3X depending on how much they are begging.

The tank is a 55 gallon with an oversize Rena canister filter sized for 100 gallons.

The other fish are an assortment of little tetras, a few small danios, a couple of little Borneo Sucker Fish and a couple of Mystery Snails.

It is a planted tank with Seachem substrate, random sword plants, a couple of onion plants and some ulvacious.

The water chemistry is a de-ionized water with a pH of about 6.7 or 6.8 and a kH of about 4.5

0 Nitrates, 0 Nitrites, 0 ammonia

I put a Poly filter in the filter to see if there was anything icky that I didn't know about and it just turned the normal light tan color for organics.

I have used several different antibiotics but it didn't make any difference in either curing one that was acting this way or preventing any others from doing the same thing.

I have tried isolating the affected angel and treating it with antibiotics, antifungals etc but they still die within a few hours.

They have all come from the same breeder who I have recently found out supplies a number of the aquarium shops and pet stores in my area.

I am at a loss and thinking I should just go back to easier fish. I have a twelve year old Parrot Cichlid and two five year old Severums in different tanks and I've never had this kind of a problem. Even in the angel tank, it's only the angels that do this. Any advice you have would be immensely appreciated.

Answer
Hi Teresa,
This is really a mystery to me.  Your water is perfect, they should thrive in this water.  The only thing I could add is maybe there is a viral disease in the pet stores fish tank, which would be very hard to find, but then, they would not last one month in your tank.  Also, it could be the breeder.  Fish like humans should not be mated if they are brother, and sister.  Some breeders do this, from one spawn to the next which eventually makes the fry very weak...I don't think a good breeder would do this, but maybe..  Seeing as all your Angels come from the same Pet Store or Breeder, I would buy an Angel somewhere else to see if this happens again.  There is a reason, we must find it.  I would also try to call the breeder, and mention this to him.  He would then look into this himself seeing as all your Angels died, not just one, but all of them.  Try to find out what water they are raised in, and if they are on dry food when he brings them to the pet store.  They should be.
The swollen tummies could be overfeeding.  Overfeeding fish, leads to constipation, which often goes unnoticed.  Swim Bladder Disease then occurs, and finally if left untreated the fish dies.  Make sure you are not overfeeding them, and provide a healthy diet.  They should have daphnia, bloodworms, a good quality flake, etc.  The more you vary, the healthier they will be, but be careful not to overfeed.
Also, there is a factor of water temperature.  The Angels should be in 80 degree water.  Cool water is not good for them, so maybe this has something to do with them dying...just wondering about your water temperature.  I keep mine in 82 degree water.
Please keep me posted if you do buy other Angels, and if they survive.  I breed angels myself, and have never heard of this problem.  I am really at a lost here, especially that your water is perfect, and that your other fish are well.
I hope we get to the bottom of this, it so sad when fish die.
Lynda