Death Of 4 Black Phantom Tetras

Hi Guys,
I hope I'm posting this in the right place. I'm fairly new to tropical fishkeeping (started in May this year) and thought I was doing pretty well, until Monday.
I've got a 90L tank, reasonably well planted which had 6 Black Phanton Tetras, 4 Leopard danios and 7 Rili shrimp. I wanted to change some of the decor and take out a largish rock that my friend gave me with the tank and replace it with some bogwood I bought. I did this on Sunday when I was carrying out my weekly water change, mainly so it would be easier to move stuff about with less water in the tank.
I did all that and everything seem fine, until Monday morning when I noticed that 3 of the black phantom tetras had died. I checked the water conditions and the nitrate/nitrite/ammonia levels were OK but a carried out a ~25% water change as another of tetras wasn't looking to well but that didn't improve things and the tetra died within an hour or so.
All the other fish and shrimp are fine and don't look ill at all, but I'm struggling to work out why this might have happened and can only think of two possible causes:
I had glued some Java moss to the bogwood and I recall reading somewhere that when glue cures underwater it can use up some of the oxygen in the water, however the bogwood was left for 3-4 hours for the glue to set and then soaked overnight before putting it in the tank so it should've had time to cure?
or
For some reason these four fish have found me moving the scenery around to stressful and that's been too much for them?
Sorry for the length of the post, I just wanted to make sure I'd covered everything I did so I can get as much advice as possible to avoid this happening again. 

Could you also let us know what glue you used.
Thanks

Hi Littlefish, Sue,

Water hardness GH = 6, KH = 4. I carried out a fishless cycle with ammonia before the danios were added towards the end of May, the black phantoms were added two weeks later so they have been in there ~6 weeks.
I use a Fluval U3 internal filter and a sand substrate.
I didn't notice any bubbles when I removed the rock, there was some silt/debris stirred up but nothing I thought excessive. I actually removed the rock Saturday not Sunday as I originally said.
On Monday the Ammonia and Nitrite were 0ppm. Nitrate was 20ppm (Tetra 6 in 1 dip test kit).
The fish didn't seem stressed when the rock was removed.
I pre-soaked the bog wood for 4 days when I first bought it 2 weeks ago, but I didn't have any Java moss at that point so I stopped soaking it. I glued the moss to it Saturday afternoon and left it ~4 hours to set before soaking it again overnight.
The glue was Gorilla super glue gel (Cyanoacrylate).
As the other fish didn't seem to be affected I chose to risk leaving the wood in the tank and the other fish have all been fine since.
I've attached pictures before and after I changed the tank around.
 

I must admit to being stumped as well.
If there was something horrible with the water the shrimps would have been the first casualties.
After 6 weeks we can rule out some disease from the shop which would have killed them faster and the danios would have caught it was well.

 

Thanks for taking the time to respond to my post (and thanks for the compliments on the tank  ) I really do appreciate it.

fcmf - sorry my mistake, I use the Tetra 6 in 1 as well as a tetra ammonia dip test.
There were no scented candles or anything external done in the house that night that would've created any fumes that could have affected the water.

The only other thing I did do that day was tweak the temperature down from 25C to 23C as I was planning to add some cory pandas this week and I thought 25 might be slightly too warm for them (that's also the reason the rock got taken out, to create a little more bottom space for them). Although it's not exactly a massive drop in temperature is it?

As you've mentioned if it was a water quality issue I'd have expected shrimp casualties as well as the fish.

I'm starting to think it might have been a combination of factors, maybe some issues with the glue, rearranging the tank along with some fish that were slightly more nervous than the others?
The 2 male black phantoms that have survived are the ones that generally occupied the centre of the tank, the other 4 tended to spent quite a bit of time hiding in the plants, so maybe they were a bit more stressed by the change in surroundings?

I got 4 more black phantoms on Friday to bring the numbers back up. They're still a little shy at the moment but hopefully they'll start venturing out from behind the plants before long.