My datnoid is sick. Hope your help

One of my small three bar indo datnoid is kind of sick. He was good at the morning and ate well, but in the noon I found it swimming bowing the head downside in the middle of tank. They have never swum in the middle of the tank before at the daytime. They always swim at the bottom and are shy to come out when I'm near. It was bit slow at swimming too, that I can even get my hand close to it in the water. Other one was bit of chasing it and knocking. I put some dry shrimps then, but this sick guy didn't eat, while other was eating. Then I add some salt to the tank and in the evening both ate shrimp well. However again now in the dusk it was much worse again and the other guy was seriously knocking this guy and it was not defending or swimming away. So I now get it separated till it will get better. I don't know what is wrong, he seems breathing very well, but just keep bowing the head downside and swims everywhere in the new apartment like a sick one. Normally they are shy and hide whenever they see me.
Is this something I should consider seriously? Hope your help. They are small than 2.5".

strollo22 said:

Do you have pictures of the sick fish? Is it swimming normally or does it look like it's lost it's balance?

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Yes it was kinda it had lost the balance like it couldn't stop swimming and its head was bowed down. Can't take pictures now. It's midnight and very dark at where the tank is. I just checked now, and it looks better now. It has stopped swimming around and bit of trying to hide when I was watching at it. Head thing also seems much better now.
I think he will be okey in his new apartment, wondering how long I should keep it here. Thinking of at least 2 to 3 days. What you think?

strollo22 said:

Yea I would keep him in there for a few days, add some salt to the water, 1 tablespoon per 5 gallons. Is there only one other Datnoid in the other tank with him? Because if so, you need to add a 3rd.

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Yes I'll keep it there at least for three days.
Yes I've only two. They are expensive here.

There are small mellow cichlids(only males except for severums) with them and two arowanas babies. There were cool for a month now, no big fight but a noticeable growth of both fishes. There is a plenty of space for both of them with lots of hiding places. 120G. I added baby demasoni malawi pair recently with a couple of another malawi babies. May be that is the trouble. May be I should get rid of them from that tank.
Sick fish is still in the new apartment, doesn't look very well yet. Head thing is now fully okey though as it seems.
Trying to hide sometimes when I come to see it, but most of the time comes out openly. When they'd been fine they were always used to hide at my scene apart from the feeding time.
Plan is to put it back in tomorrow noon and observe, if it continues to behave abnormal, I'll get rid of malawi babies first and put the sick one back to new apartment for few days again.

As all have said I think it's an injury or stressed by malawis.