Loosing Lots Of Fish

I have  been losing a lot of fish since last week.  My problems started last week when I lost a blood fin tetra. It had a humped back but nothing else obviously wrong with it. I have had it for a long time so I thought it had died of old age. 

But since then I have lost 3 neon tetras. Another 1 is missing and another 1 is not looking good.  Before my neons died their bodies were leaning slightly sideways as they were swimming and they kept resting on the bottom of my tank. One had a bit missing from its tail fin but I think it was being bullied when it got weak.  My ill neon has the same problems.

 None of the neons have any wounds, fungus or signs of disease on the bodies and apart from the symptoms above there are no obvious signs of what’s wrong with them. My tank also has 3 cherry barbs and 1 Lampeye Panchax which are not showing any symptoms.

I have put some Anti Internal Bacteria medicine into my tank twice (Once Last week and once yesterday) in case the fish had  a bacterial infection.  Before I put the second dose in I did a 50% water change.  During the water change I siphoned my gravel and it seem quite clean. There was a small amount of algae on my plants and glass which I have now removed.

I’m struggling to work out what could be wrong with my fish so I don't know what to do next.  Does anyone know what could be causing the deaths?

Thanks in advance!

Tank stats:

Size: 48l
Been Running: 3-4 years
Filtration: Under gravel filter connected to an air pump (been in place for several years)
No3 : 0
N02: 0
Hardness: 4d
PH : 6.4
CL: 0
The  water stats were taken before the water change.

Fish in tank:
3 cherry barbs (All ok: Been in the tank since last year)
2 or 3 neon tetras ( 1 missing, 1 ill and 1 ok. Been in the tank for 2 -3 months)
1 Lampeye Panchax (All OK: Been in the tank about 5 months.)

I can't be much help, except to say I'm in a similar situation at the moment. My tank's been up and running for a few years with cherry barbs, 5-band barbs and Lemon Tetras, and I put 7 Diamond Neon Tetras in in March this year. The first neon faded and died in July and I've lost one per month since then. One day happy, next day the red has faded and they're hanging around the back of the tank, drifting in the current. I haven't been able to work out why.

Could neon tetra disease be a possibility (although I think there would normally be more noticeable external signs)? Also, it's probably of no relevance but is your NO3 (nitrate) reading definitely zero? Nitrate rarely hits 0 as there is almost always some nitrate in your tap water. Did you mean NH3 (ammonia)?

Are you using strips to test the water? I notice that you gave a reading for Cl and strips usually test for this but not ammonia. If you are, can I suggest getting at least ammonia and nitrite liquid testers so that you 1) find out your ammonia and 2) find out of your nitrite really is zero as strip testers aren't terribly accurate.

You're right. The test results were taken using a test strip. I have a ammonia liquid tester. I tested the water for ammonia today and it was 0. Its only a few days since I did a water change so I will test again tomorrow to see if the ammonia starts to rise. I will buy a nitrite liquid tester.

Do the neons show any changes at all? No colour changes - bleached out red, blue area looking yellowish? These are both symptoms of neon tetra disease, which is incurable unfortunately. Other symptoms of this include swimming oddly, bent spine and pointy lumps on the belly.

I didn't notice any colour changes on the dead fish and there are no colour changes on the remaining neon. Before the neons died they were leaning slightly to the side as they were swimming. The blood fin was swimming normally before it died. The first fish to die (the bloodfin tetra) had an arched back. None of the neons had arched backs. None of the fish had pointy lumps on the belly.

The remaining neon and the rest of the fish still not showing any signs of illness.

Isn't the swimming sideways something to do with the swim bladder? I know it normally affects goldfish and betas but it can affect a lot of fish.

Yes swimming sideways can mean swim bladder problems. My fish were only leaning slightly to the side rather than swimming completely on their side. Is it normal for swim bladder to kill so many fishes so quickly?

Remaining fish still ok.  Will get a nitrite test kit tomorrow just to see if my water has got too much nitrite.

Since your water conditions are fine and it's just the neons, I would just put it down to them being weak specimens. It does happen that a shop gets a weak batch in, something that is notorious with fish like guppies.


On the subject of water conditions, it is unusual to have zero nitrate unless the tank is heavily planted. With liquid reagent nitrate testers, all the makes have one bottle that the instructions say to shake well. Can I just check that if your tester said to shake a bottle that you did do that? One of the reagents in one of the bottles comes out of solution and drops to the bottom of the bottle. The shaking is to redissolve it.