Pictus Catfish Oddly Dying

[Westeros_Coast]

Today I lost one of my pictus catfish and I have no clue why. It started only this morning not being able to control its body, floating on its back and sides bouncing into the sides and driftwood. about 2 hours later, it had died. Now my other pictus catfish is doing the exact same thing. It's just floating upwards not controlling its body, excessively gulping and getting pushed around by the current.
 
I've had these catfish for about a month, and they've seemed perfectly fine and healthy. Two days ago, I added 2 dwarf gourami. Yesterday, i added a new piece of driftwood and rearranged the tank a bit. I also just started adding flourish excel, but I doubt this is the culprit. These are the only things I've done recently. I figured the first one died from swim bladder disease, but now its happening to the other one hours later.
 
I can't see anything wrong with the other fish. My second thought is maybe the new gourami have white spot, but I don't see anything visibly wrong with them. Should I treat for white spot just in case?
 
My tank is 37g.
 
Current stock is:
4x Longfin Zebra Danio
1x Bamboo Shrimp
2x Dwarf Gourami
1x Pictus Catfish
 
I tested the water parameters and seems fine.
pH: 6.4
Ammonia: 0 ppm
Nitrite: 0 ppm
Nitrate: 5 ppm
 
When I added the new driftwood, I had done a 60% water change. Today, when I found the first catfish dead, I did a 75% water change.
 
Any help is greatly appreciated! I really don't want my other catfish to die too

 

[Efishent]

Are they swimming upside down? I had a goldfish 2 years ago that was always swimming upside down, and it turned out they had an infected swim bladder. I can't be sure though, I'm not an expert. I hope you can make it better!
 
 
 
EDIT: The awkward swimming position is most likely the fault of some swim bladder disease. I have read that this can be caused from shock, parasites, bacterial infection, physical damage, and deformities. There are probably more reasons I don't know of, too. If it is a swim bladder disease, you can treat it, but you will have to figure out what kind, first. 

 

[WILDER]

Sorry for your loss. R.I.P.
 
How are the other fish doing?
Did you soak the driftwood before adding to the tank?