Angels dying

[nascar]

I really like angle fish but I don't seem to be having much luck. I keep them in a community tank and everything else seems to be doing well. My ph is 7 and I change 20% of water on weekly basis. I also keep the water at about 78 degrees. About every 2 weeks one of them starts swimming sidways or upside down then all of a sudden.... done. The other 3 seem to be doing well. Can anyone help me here.

 

[Shizat]

Whats the size of the tank and whats your water parameters as well as other fish in the tank?

 

[nascar]

20 gals and I have some black tetras some neons 2 guppies and a couple albino catfish.