sick angel fish

Hello, I have a 29 gallon tank, and keep 5 angel fish. One on my fish has stopped eating and lost his/her energy to swim around the tank with his fins stretched out. He's been hanging out at the bottom of the tank, hasn't been socializing with the other fish. He hasn't been showing and physical abnormalities so I'm confused on what may be wrong with him.
I change the water and check the ph level on a regular basis. I feel like I take care of them really well I don't understand why any of them should be sick.
Danny thank you

Hi,

How long do you have the angels? Were they all bought together?

three months and no they were not all bought together

Angels are very sensitive so it is possible that one of them brought something with it from the store, I usually recommend 6 weeks quarantine time to make sure they are in perfect health before introducing them to the main tank.
Still after three months it is possibly something that has to do with the water.
Do you inject co2, do you monitor nitrate, there are other variables beside ph.
Can it be that he is not sick but just have different behavior?

Ive been watching them, since i first sent the email. It seems like the other fish are very aggressive with that one fish. They chase him around, so i think he's just scared of the other fish.

I suppose there is no way to avoid territorial fights altogether but it is not fun to see one of the fish isolated

As angelfish become sexually mature, they also become territorial and aggressive. Eventually, you may get a spawning pair, and they will intimidate their tank mates sometimes to the point of causing death. A spawning pair can also be quite aggressive with each other. Right now in a 45 gallon tank, I have a newly formed pair of angelfish, 20 or 30 of their babies, and three more mature angelfish hiding in some plastic plants. I'm going to remove some of the plants so I can catch the extra, mature angelfish and transfer them to another tank. That's not going to be easy, and in fact raising angelfish is not always easy. On the other hand, angelfish are always fascinating and worth the extra effort....if you're a nut like me.::

Thank you guys, my fish is still being isolated, but at least now I know he's not sick. Im just gonna get a divided to give some personal space for him. maybe when he gets a little bigger ill take the divider out, as of right now ill just let the more aggressive fish fight it out.
danny

Good luck, and let us know how all the angelfish are doing.