Sick Dwarf Rainbow Fish

Hi everyone

One of my Dwarf Rainbow Fish is sick / dying.  It suddenly started doing tumble turns and is now head down, still breathing but struggling to move.

It's a fairly new tank and the Rainbow Fish were only added about three weeks ago after cycling the tank with Zebra Danios. For reference, it's an 80cm, well planted tank. I have been doing regular water changes and testing the water.  Tested it just now and it all appears ok.

Is there anything I can do?  Or what is the best way to put it out of its misery?   

Postscript:  It has actually died but I would welcome any thoughts / advice.  Is it a one off or are the rest at risk?  What should I be checking for (other than ammonia, nitrites, nitrates, pH)? 
And what do you do with your dead fish?

Hi thanks for repying.  Will try and answer your questions.
The water is soft. The tank has gravel in the bottom and is well planted with plant plugs, which are growing well, and it has quite a few different rocks, a small flower pot and a decorative cannon.
 
It had just the plants for about a week before the danios went in. I am using a previously used pump/filter -  I actually cleaned everything before they went in the tank and then wondered if I shouldn't have. 
 
When the danios went in I was testing every day and getting negligible readings except for one slightly elevated nitrites on one day.  I read in a reply to someone somewhere on here that if you have plants in the tank and are getting stable readings you can go ahead and add more fish. So the rainbowfish were added about 4 weeks after the danios. I didn't take the danios out - should I have? I don't have another tank. 

The readings since have all been stable and negligible. I've had a couple of slightly elevated nitrate readings - just around the 5ppm -  and have been doing weekly water changes of about 10-12 litres.  The pH is currently 7.4 (with the high range test)

There are 6 zebra danios, and there were 6 dwarf rainbowfish.

I did have a look at the dead fish and can't see any marks or anything on it.  It all happened really suddenly.  Spent a long time watching them yesterday and they all looked fine. Was just thinking about thawing some food for them today and saw one rainbowfish kind of gulping,(opening and shutting it's mouth like it was eating but there was no food there).  By the time I'd got the food, the fish had started tumbling about and it must have died within two hours. (I think it was the same fish!)

Will have a look at the article, thank you.