Clown Knifefish Help

Question
Hello,
We have had our clown knifefish for a little over 2 years in a 150 gallon freshwater aqurium. 2 days ago, I noticed he doesn't like to come out of his castle (his favorite place to be in) he would just eat whatever floated closest to him. My daughter (10 years old) is worried sick!
I noticed today however, that he comes out but is swimming kind of vertically and towards the mid end of his body it is rigid and stiff... it looks to taking a toll on him to do so. I do have one other knifefish and he is doing fine.
There are 4 others as well, but they have been altogether since we brought them home.
My daughter thinks I should take out the castle cause she thinks he is too big to stay cramped up in there, although the castle is bigger than him. I'm not sure what to so! I could really use your help in this. I am calling everyone I think of that could help.. but right now it's 7 in the am where I'm at.
Thank you for reading.

Answer
Hi Sandra
Don't know how much help I can give, I've always stayed away from buying clown knife fish, because of how BIG they end up getting lol.  I do have a few questions for you though, maybe we can figure this out.
 
1.  How big is the "sick" fish, and how big are the others?
2.  How big is the area inside the castle he hangs out in?
3.  Do you feed feeder fish, if so where do you get them, and what else do you feed?

What I'm thinking, CK's as they get older, get a bit more territorial.  He may be hiding out in the castle because the other's are picking on him, or he's staked that as his claim and he's protecting it.  That's partly why I was wondering about their size.  They're also usually nocturnal, so he may be coming out when the lights are out in the house.   Regarding their territory, your tank should be ok size wise for one, 6 though is way too many-unless I'm misreading what you wrote :)  He may be stunted as well if there's that many in the tank.  Fish will stop growing if their surroundings are too small for them, there's a hormone that gets released in their bodies that tells their bodies to stop growing.  The problem is, their insides/organs don't stop growing.  They usually end up with a distorted body shape from their insides being crunched together, and their life span is dramatically shortened.
 
Regarding the castle now.  It may seem big to you, but he may be cramped inside it, but that's his territory so he just makes do with the size.  You could try removing it, and replacing it with something larger for him to fit in.  They love pipes and tubes, if you can find one big enough for him to easily get in and out of.  If you do remove it, be sure to rearrange the rest of the decorations and such in the tank.  This will help cut down on aggression while they all reestablish their new territories.

And, the feedings.  CK's love live food.  I've heard/read a lot of stories about some that's all they'll eat.  Unfortunately, feeder fish aren't really the best thing to feed fish.  They usually end up messy in the tank, which causes poor water conditions/stress & diseases.  Feeders from the pet store are usually not kept in the best of conditions, because they're food.  So, there's a high likelihood of spreading disease into your tank from them.  For feeders, it's recommended to breed your own.  Not sure what you're feeding, but frozen foods like brine shrimp, bloodworms, beefheart, etc. are good.  Live worms are good as well.  Since they're nocturnal, I'd also recommend feeding them at night, after the lights are out.

That's all I can really think of.  If it doesn't sound like anything I've suggested, and you think he's looking sick, I'd quarantine him in a separate tank immediately.  And, if you do have all 6 in one tank, I'd give some serious thought about rehoming some of them.  Either with a new tank(or pond lol) or possible even giving some away.  

Hope at least some of this helped you!  Good luck, and let me know if I can help anymore!!

Christy