I have 4 29 gallon tanks just for angel fish.. I have 5 21/2 year old marble angels in one tank that has been established for 3 years with live plants and a co2 system.. I have never lost a fish in this tank.. 4 months ago 1 angel was found laying on its side on a rock and 2 weeks later a second angel started doing the same.. They are boyh still alive and both struggle to the surface when I feed them.. They are still alive but lay on their sides all day occasionally moving to a new location.. The other angels leave them alone it is strange.. I have not added any fish or plants to the tank in over a year and only feed flake food??? Does anyone have any feed back.. I have raised and breed fresh water fish for 30 years and have never seen any thing like it.. The tank is 80 and the ph is 1 point over neutral the ammonia is never present??
what is the ph level of your aquarium?what's the nitrite, ammonia,nitrate level of your tank...what do you feed to your fish?
Angelfish usually do better at 82 degrees. Living in cool water tends to stress them over time. Just guessing, the most likely source of your problem seems to be the carbon dioxide system. Plants exhale carbon dioxide when the lights are off, and too much of the stuff can cause rapid swings in pH. Angelfish can thrive in a wide range of pH values, but they cannot tolerate rapid changes.
Do your aquariums get regular, partial water changes? You may want to do a fairly large partial water change and see if this helps the angelfish.