Giant Danio swimming vertically

I have a 38 gallon tank which contained 5 Giant danio's. A week ago, I had discovered a fish dead, with pretty much its stomach exploded and tissue all over the tank, I have no idea how he died, since he was fine the previous day before I shut off the lights, but next morning, not so much. I immediately did a massive water change, probably alittle more then 50%, anyways, soon after, another one of my Danio's started swimming vertically, at the bottom of the tank, with his head facing up. He often stays in the same exact spot and treads the water in this position. However, when it comes to feeding, he has no problem, swimming and fighting with the other danio's for food. Can anyone tell me what is wrong with him? (attached photo's)

---Additional info---
The tank has been running for about 4 months
I do a 25% water change weekly
I dechlorinate the water with Seachem's Prime
The filter is cleaned out monthly, (media rinsed in bucket filled with tank water)
Ammonia: 0ppm, Nitrite: 0ppm, Nitrates: 20ppm (its water change day )
pH: 7.5

 

I am thinking about what could have happened and can only come up with some sort of infection from the dead fish. I personally do 50-75% water changes in all my tanks every two weeks. Don't be concerned about doing huge water changes. I would probably recommend a huge water change using Amquel+ or the products you use, wait 24 hours and test the water. Your parameters are fine, but don't test for water infection. I may be wrong, but whatever caused that fish to 'explode' may have contaminated the tank. I would also take out the decor and clean it well. Tank shouldn't go through a cycle as your filter will remain intact. Can you also vacuum the substrate really well in case there is dead tissue there?.

 

I always take out the decorations with each water change, and let them soak in hot water for the duration of the change, then rinse them, and put them back in the tank, Also, i'm currently using sand, but when I cleaned the tank from the "exploded" fish, I definitely went over every inch of sand, and picked it up any and every debris that I saw on the substrate.
I did the massive water change because I was afraid of contamination, though I find it odd that only one out of the 4 fish are having this problem swimming. (Also I forgot to mention, but I run an airstone too, so there is definitely no problem with oxygen)

 

It really is a mystery.... I hope someone else has an idea about what could be going on. You don't have to take the decor out when you do water changes, many of the beneficial bacteria live on the decor and become part of the tank-filter. I never touch mine really unless I am doing major tank changes.

 

i'm really starting to worry about this fish, google has turned up nothing for me.