Black neons dropping off !

HI guys and girls
I am trying to figure out what is going on with my black neons, firstly I am an experienced fish owner have been keeping fish for about 13 years now from African Chichlids to SA, CA and Australian native setups.
With tank setups from 40 gallon right up to 500g.
I know all about the nitrogen cycle, water parameters etc.
The tank I am having the tetra issues with is a 40G with a 20G sump loaded So a total system of 60 gallonwith jap mat, sponge, 12lb of mixed ceramic media and a sponge filter in my return section of sump.
Ammonia = 0
Nitrite = 0
Nitrate = 10
Nitrates never go over 10.
Tank has low light plants, annubias, Java fern and swords.
10 Beunos Aires tetras
8 False jullI Corydoras
4 electric blue Rams
3 otos
And now down to 5 neons after loosing 5.
The system works really well, the BA's being at a school of 10 leave everyone alone they just school and follow each other around and I have never seen them bothering other fish.
The corys as you would know are very peaceful and do there own thing and the Rams
have there own section in amongst the annubias and are doing great no fighting etc.
But my black neons which never get picked on and also do there own thing are beginning to drop off.
The water is well oxygenated also being a sump I have it dumping onto a drip tray causing a lot of oxygenation and also the sponge filter in the sump return which cause a lot of surface agitation.
Water changes are done every Sunday and Wednesday.
30 percent with a gravel vac on Sunday's and just a 10 percent with no gravel vac on Wednesdays.
I never miss my water changes they are like clock work.
I feed my fish once a day and never over feed I feed them HikarI micro pellets, NLS flakes,
Occasionally hikarI freeze dried brine shrimp occasionally Today tubifex worms.
The corys get HikarI wafers the catfish type each night with lights out and the otos get squash, zucchinI and cucumber every other day.
I am sorry I cannot get photos of my neons until later today as I am now at work but they are very bloated and breathing rapidly I have tried holding off on food etc with no change to there bloatedness.
The bellies are evenly fat all the way around.
All my other fish are doing fine and have no signs of anything wrong at all.
It is just the tetras.
To explain what he tetras look like without a pic is like they have swalloed a small marble.
Any ideas anyone ?

 

Are the tubifex worms freeze dried? They can carry nasty parasites like callamanus worms if not.
Are they still passing feces? Normal feces?
Do they eat? Spit out food and swim under it?
Before death, is there any other syptoms? Red dots/lesions, tail rot?
You could take them out ad put them in a separate tank or tub/whatever with a healthy dose of Epsom salt and don't feed anything for a few days, for now. A bare bottom Ten gallon with a simple airstone or spoonge filter and heater would be nice (if you had one on hand)
I am suspecting Fish Tuberlosis infection, or callamanus worm parasites...... but I could be overreacting, I tend to do that!

 

They pass feces but I would not say at a normal rate and the feces is not a normal shape, the tubifex worms are freeze dried,
They eat fine no unusual feeding they are actually ravenous and feed normally. Also no unusual signs before death other than constantly breaking rapidly.
No lesions, dots or tail rot.
They look perfectly healthy and have really good colour just the bloated belly and rapid breathing.
I am thinking its a bad batch of fish as I have only had the black neon tetras in there for 2 months.
Unfortunately I have no quarantine tank, that's one thing I have never had and should get.
And I can't salt the tank as I have otos and corys.
And really don't want to medicate tank I hate doing that as it crashes filter but I may have no choice ?
I have never had issues with disease before besides some white spot and I usually just use the salt and temp treatment.

 

Could this be another case of Neon Tetra disease?

 

Unlikely, without other symptoms, I honestly don't know what's making his tetras bloated.
If they were recently added, I have seen mycrobacterium infection cause severe abdominal distress bloat and death.
Neon tetra disease presents as a brownish under the skin lesion near the cadual pendacule

 

I have been feeding them shelled cooked peas for the last 2 days and nothing else, seems to be working. Will keep posted.