Fish dying one by one

[ehh]

About two weeks ago I lost one of my ruby barbs. From day to the next she became reclusive, her belly became slightly swollen, and her anus looked irritated. Les then 24 hours later she died.
A week later the exact same thing happened to another one.
I assumed it was age. I've had them for a year a half. I know that's a short time frame but everyone else appeared healthy. Water tested fine.
Well I just lost another one in the exact same fashion.
Everyone is eating and is swimming around. This illness strikes over night, death is within 24 hours and it is slowly spreading throughout my tank.
Any advice?

 

[ehh]

From what I've observed I've seen no stringy poop. But like I said it happens so quickly.
However, i have the latest one that died a little squeeze and a lot of white/clearish liquid shot right out of her anus.

 

[HumanBean]

What are you feeding ?

 

[Yuki Rihwa]

Happened to me from time to time (other barb species) but I was assumed its died of old age and my barb age range from 2 to 3 years old. They were happy swimming around the tank like usual and just died overnight one by one but NO EFFECT on Juv. or younger barb less than 2 inches size, If there was a cause to their death I would love to know and preventing it happen again.
Edit:
Diet consist of Bloodworm, New Life Spectrum Micro pellets, cut smelt and all they can eat veggies in planted tank

 

[ehh]

So far my really old bn pleco has shown so signs of illness. Not my old catfish.
I'm feeding them nls algaemax. They get feed every one or two days. They get frozen stuff like mysis or brine shrimp once a week.

 

[ehh]

I have three other species of fish in there and none of them have died yet. only my little ladies have been dying :-(

 

[Yuki Rihwa]

Yep, seem only happen to Barb species, I have more than 6 different species in my tank and only barb died from time to time and no one else show any sights of sickness. I got 30x Ruby barb arrive this weekend, i'll monitor them closely to see if they have a same problem.
*If there a cause I would assume the problem come from/with captive inbreed species like these popular barbs.

 

[ehh]

im having zero luck finding anything that matches up these symptoms. im kind of hoping it is something specific to the barbs. a couple of the fish that are in there ive had a long time. one of them since i started keeping fish.

 

[Wailua Boy]

I would suspect a internal parasite, hard to say exactly what. Sunken stomachs in two separate fish is a little suspicious. Most farm raised barbs are likely to of picked up something along the way. Might try the metro/prazi combo


Do these fish get their fare share of food? Do they have an appetite?

 

[ehh]

none of the fish had sunken stomachs.
yes they get plenty of food.
they have an appetite until the day they die. i feed them two days ago, they all ate. one got sick yesterday and died today.
right now im trying prazipro.