Angel fish dying

Question
Hello-
I have a 20 gallon planted tank, nitrites and amonia are around 0, my pH is about 7.3, I fertalize with co2 and potasium: I have 1 pearl gourami, 3 neon tetras, 1 candy pleco, 1 otto, 3 african dwarf frogs, and I am loosing 3 half black angels.
My question is in regards to my angels of which two have now died and the third looks like he is going soon. I first noticed the problem yesterday in the largest one. He was fine in the moring, no signs of sickness other than the fact that they seemed to have been hanging close to the water surface lately (but not gasping for air). When I got home at night the big one was sideways on the ground barely breathing, his top spine area had a yellowish color to it, and he seemd pale, one eye was glazed over, and he soon died. I noticed that the other two seemed to have white spots, so I was thinking ich, and I turned up the temperature and started medicating. This morning when I woke up, another Angel looked like the one that had died before, and he died by noon. The last one had a yellow tinge around noon, and tonight he has just started swimming eraticly. The only thing else I noticed was some stringly white feces on one of them yesterday, and the smallest ones tail looked like it had been bitten away at. It looks like it is too late for these fish, but I was wondering if you might be able to tell me what is killing them so that it doesn't happen again.

Thank you very much-
Mike

Answer
Mike:

Angels are slow moving, gentle critters.  Any change in water chemistry will kill them quickly.  How often do you do your water changes? You should be doing them about 25% every two weeks.

What is your water temp?  You have given me symptoms that could cover many illnesses, especially the white spots, which may indicate ICH but would spread rapidly to your other fish.  Have you added fish to your tank lately?  Your ammonia and nitrites shoud be at 0ppm all of the time.  

Finally, don't have frogs in your tank.  They are cute, but they are not compatible with tropicals, especially angels.  

In conclusion, please buy one of the many wonderful books available on keeping tropicals.  

MJ