My fish died!

Question
Last night, I changed 1/4 the water in my angel fish's tank to try to get rid of the algae.  I had two in a 20 gallon tall tank.  The next morning, both were dead, and one was bleeding from the gills and fins!  I tested the water after I found them to try to figure out what went wrong, and I couldn't find anything wrong:
Nitrites: 0mg/L
Nitrates: 0mg/L
Ammonia: 0.1mg/l
pH: 6.9
Temp: 72F (21C)

They were just fine last night.  Any ideas about what happened?

Answer
Hi Leah,
I'm sorry to read about your Angel Fish.  It is hard for me to know exactly what happened because I don't see your fish.  One thing I do know for sure is that your water is too low for Angel Fish, and this could have been a big factor.  Angel Fish must be kept in 80-82 degree water.  The water was too cold for them, and this may have contributed to what happened in your aquarium.  When tropical fish are in cool water, they are cold, and stressed, and develop disease.  I have a strong feeling that this was the cause.  Check your food to see if it isn't outdated.  It is always a mystery when fish die suddenly.  
Have you checked with your local water company?  Maybe they added some toxic chemical that would be bad for your fish.
This is all I can tell you, but 72 degree water did play a big role in their sudden death.  Apart from the cool water, your PH, and nitrates, and Ammonia were perfect for them.  
Hope this helps
Lynda