Ember tetra has a large swollen belly

Tank
15 gallons, 3 years old with filter and heater, temperature usually at 24 C.
Stock is 6 Ember tetras and two dozen cherry shrimps, two nerite snails.

Maintenance
25-50% water change once every 2 weeks.
I use Tropical Aqua care as dechlorinator.
I rarely vacuum the substrate.

*Parameters - Very Important
I did cycle the tank but I do not have any test kit so I don't know what are the parameters, nor can I get one.

Feeding
I feed once a day or every two days, I only use fish flakes and I just sprinkle a pinch.
I use shrimp pellets for my cherry shrimps, 3 pellets every 3 days.

Illness & Symptoms
I've had my Ember tetras for over a year now. I noticed the symptoms one month ago. One of my Ember tetra has a swollen belly, almost like blown as a balloon, her scales are not sticking out, she can't swim properly, but she has no problems eating and doesn't look like she's in pain. A few months ago one of my Ember tetras jumped out of my tank and fell on the floor, I put her back in the tank and she seemed fine afterwards, I am not sure if this is the one however, also worth to note that one my tetras died 2 month ago, unknown reasons.
I did not start any treatment for it.
None of my fish appeared ill upon purchase.
The fish started hiding more than usual.

Explain your emergency situation in detail.

 

Wow, that's huge. I'm thinking it's a tumour given the size of it.

 

Wow, that's huge. I'm thinking it's a tumour given the size of it.

It's transparent, there's nothing inside that huge bloat, almost like just water.
Is there anything I can do about it? If not, should I put it down or just leave it be?

 

It's transparent, there's nothing inside that huge bloat, almost like just water.
Is there anything I can do about it? If not, should I put it down or just leave it be?

What I would do (and this is just me) is to isolate the fish in a quarantine tank. I would do a daily epsom salt bath (a relatively gentle treatment for dropsy), to see if the fluid retention eases. Here's a tutorial on using salt.

. I'd do this first before considering whether or not to euthanise.

 

The epsom treatment is very likely too late. The fish has already a spine deformity and the abdomen is swollen to more than the fishes normal size. I'd put it down. That's less stress for all involved. Fish and owner.