As others have said, a lot more information is needed here to be able to give a helpful answer.
In addition to answering the other questions asked above, what size are your discus, and what is your water change routine.
The number of discus you have, (and any other fish you have) how long you've had them, the size of your tank, what kind of set-up you have, and details of your feeding will all provide clues as to the problem.
I have a serious problem with one of my discuss. He is as old as the other five yet he is still only three inches long while the others are five inches. He hides constantly but not being attacked by anyone else. He is dark and will not eat. I am concerned about putting him in the isolation tank. I don't know if he could handle the stress of the transfer. I am at a loss of what is happening with him. I really really need your opinion. Please help. I'm still having a little trouble figuring out how to start a new thread so if this post is in the wrong place please forgive me.
Sorry, but you will need to give us a lot more information before we are in a position to help you.
Answer these questions to give us some clues as to what the problem may be:
- What size of tank are your 6 discus in ?
- How long has it been set up & how long have you had the discus in it ?
- Did you get the discus all at the same time, from the same source, what was that source, and were they all the same size when you got them ? What size was that ?
- What is your water change routine, how often do you do them, and in what quantity of water volume ?
- Is your tank planted & what substrate do you have in it ? What is your feeding routine & what foods ?
- What is your tank cleansing routine ? Tank temp ? pH in your tank, & out of the tap if you use tap water for WC's.