Exploding Bellies - Danios - Help Please

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Tank size: Juwel Reckord 120 (120L)
pH: 7.2
ammonia: ?
nitrite: (NO2) 0
nitrate: (NO3) 100
kH: 6
gH: >16
tank temp: 24 C

Fish Symptoms : Tank has Danios (leopard x6 and Zebra x4), Neons (2 sorts, approx 20 total), One small shark (Black with red tail) and a couple of small sucker fish.
The zebra danios have one by one shown a protruded belly that then erupts through the skin in the form of a red lump. The skin then splits apart and the belly of the fish (normally on one side only) is exposed, then the fish dies (assume when this ruptures). No other symptons. All other fish are OK. Have lost two and third now has pronounced belly and other is looking slightly bloated. Help!

Volume and Frequency of water changes: 15 to 20 % every 10 days. Have increased recently and done 3 changes in the last week to try to sort the problem.

Chemical Additives or Media in your tank: None. Have just bought some anti bacterial stuff to try next

Tank inhabitants: ?

Recent additions to your tank (living or decoration): None. All rocks and plants are quite old. Quite a lot of algae growth on rocks though. Could this be a factor?

Exposure to chemicals: None

Digital photo (include if possible): attached views of both sides of same fish and of general tank view.

Please help if possible. Thanks

Squat

I would put the fish out of its misery.
Is it a lump or organs.
What do you feed your fish.
Have any fish ever looked really skinny with a bent spine.

Your nitrate readings how.
How often do you do a water change and gravel vac.
When did you last rinse a sponge in old tank water.

The fish can bloat up causing ruptures.

Take a look here it might help.
Though it's pretty gross.
http/thegab.org/Articles/GoldfishInternalAnatomy.html

Hi Tweety

Thanks for the esponse - answers below:

Wilder said:

I would put the fish out of its misery.
Is it a lump or organs. - Looks like organs from inside rather than a lump.
What do you feed your fish. - Flake and also Daphnia or Tubiflex and treats twice a week
Have any fish ever looked really skinny with a bent spine. Only a while back when some of the original danios got to 2.5 years old. I put it down to old age.

Your nitrate readings how. - How? Do you mean high? Yes - how do I get it down?
How often do you do a water change and gravel vac. - As in original post once every ten days but am increasing at the mo to try to help the fish
When did you last rinse a sponge in old tank water. - Never done this? what does it do?

The fish can bloat up causing ruptures.

Click to expand...

Tubilex live or frozen.
I don't feed my fish tubilex it's to risky here's why.
http/www.gsas.org/Articles/1997/tubifex.html
Danio as they get older yes the spine can bend slightly.

I would stop feeding the tubilex worms and see how your fish go.

Remove a sponge from your filter only one, and rinse slightly in old tank water to fetch your nitrate reading down.
Don't do more than one sponge as it can send your tank into a mini cycle.

You wait at least two weeks before you rinse the other sponges in tank water.

When a zebra danio is nearing the end of its typically two year lifespan, there will be a moderate to severe curvature of the spine.
Taken from this link.
Didn't know they only lived two years.
http/www.aquarticles.com/articles/breedi...bra_Danios.html