Bloating death

[TwoFace99]

Well, I just had one of my Lake Kamaka Rainbowfish passed away. I literally saw their body a few minutes ago after waking up. I have noticed bloating from that one Rainbowfish yesterday, and little to no desire to eat, but swimming normally otherwise. None of my other inhabitants are experiencing any illness or odd behavior like that one did. 
I called my LFS and they suggested using two boxes of Maracyn Two over the course of 10 days, since I have a 55 gallon tank.
I notified them the only change I made recently was performing a 50% WC on Monday. I try to be as close as possible to the exact temperature rather than starting off higher and let the temperature die down to my 78.F setting. 
Any more suggestions on what to do are recommender and encouraged. Here are some photos of my tank currently. 
 

WARNING: Dead fish image below

 

Water parameters: 

pH: 7.8

Ammonia: 0ppm

Nirite: 0ppm

Nirate: 40ppm (mostly from Easy Green dosage)

kH: 5dKH

GH: 10dGH (Wonder shell supplement)

 

Inhabitants: 

2 Marbled Angelfish

2 Pearl Gourami 

9 Harlequin Rasboras

4(was 5) Lake Kamaka Rainbowfish

Apistogramma Cacautouides Pair

2 Bolivian Rams

2 Reticulated SAEs

3 Amano Shrimp

 

[Colu]

What I would do as no other fish are showing symptoms I wouldn't treat I would monitor them closely for next couple of weeks give an update if you notice any odd behaviour such as bloating rapid breathing hanging out near the surface lethargy loss of appetite @TwoFace99