Tetra and Cory Mystery.......

I keep a 36 gallon bowfront with my 10 year-old son, which has been a very rewarding experience. We have been keeping a three-spotted gourami, 4 cory cats, and 8 red-finned tetras; we use an Eheim Classic external canister rated for a much bigger tank. Two weeks ago we purchased another 4 cory cats, which has been great - they shoal, play, and are the clowns of the tank.

My son also had 2 very large pieces of driftwood in the tank. He decided to remove them since the tannin levels were making it difficult to see the new (small) corys. This was accompanied by a 75% water change to clear the tank up a bit. This was done a few days after adding the new corys.

A week later I found a dead tetra. There were no apparent symptoms at first, so I ran all the usual tests using my API Freshwater Kit. No major changes to speak of - all levels checked out daily. I chalked it up to possible shock due to the extent of the water change.

A day or two later, another dead tetra. No symptoms. Uh oh. More water tests - nothing. More continued to die throughout the week.

A few days go by, and I see my first symptom in one of the tetras - it had turned very dark and was visibly struggling to breathe. I pulled it out and placed it in our hospital tank, and tried to treat it with Ketaplex. No dice. Died the next day.

A few days ago my son noticed that the gourami had a pink lesion around its mouth. Otherwise it is behaving normally. I pulled the gourami and the rest of the tetras out and started treating them all with the Ketaplex. Another tetra, who started to darken after moving, died yesterday. The gourami and the other tetra (only one left) are eating fine, and the tetra looks perfectly healthy.

Here's the thing - the corys are absolutely fine. No symptoms, no deaths - nothing.

I have a few thoughts on what could have happened, all of which are connected to the new corys. I'd like to hear what you guys have to say, and what you all think I need to do to my bowfront. In case I wasn't clear, all medication has been administered in the hospital tank. Thanks in advance .

 

Welcome to Fishlore

Can you provide your exact water parameters - pH, temp, ammonia, nitrite and nitrates? When was this tank set up?

 

PH is 7.2. Ammonia is 0. Nitrites 0. Nitrates are 10. Temperature is 80F. After thinking about the nitrate level, I forgot to mention that I started doing 20% water changes every day once the tetras started to die to reduce any adverse water conditions that I was missing.

The bowfront has been set up since November; the gourami was moved from an older tank, and we've had him for about 18 months. The 4 corys are also from the old tank. The hospital tank was set up at the same time as the bowfront.

 

Can't think of anything other than something the corys may have introduced since that would have been the only recent change to the tank. I don't know that removing the driftwood would have been the issue (you could add carbon to remove the tannins, btw).

Prior to the 75% water change, how often and how large were your water changes?

 

My son and I maintain the tank weekly. Typically, the water changes are 2-3 gallons or so, just enough to vacuum the top of the sand substrate.

 

If you were only changing out 2-3 gallons weekly and then did a massive water change, I'm wondering if that may have had something to do with it? Though I would have expected the death to be almost immediate and not have a die-out several days later. What were your nitrates before usually?

And rather than the 20% daily that you're doing now, I would stick with 30-50% weekly water changes going forward. The pink lesion on the gourami (if you can also get a pic of that) might be the result of not enough water changes.

Hold on for more responses as someone else may know or think of something.

 

Thanks.

My major concern is possible TB or septicemia, and what I would need to do to my bowfront if I suspect that either is a possibility.

 

I don't know enough about those diseases but from what I understand, TB is very rare.

I'm going to say the tetras dying may either be something the corys introduced or the shock from the sudden large water change.

 

Some photos. The first photo is of the healthy-looking tetra for comparison. Please note the new whitish mark on the top of the sick tetra's head (3rd picture) and the whitish mouth of the tetra in the last photo. Thanks.

 

Looks like something that may have developed over time. Maybe DoubleDutch will know.

 

The darker tetra died tonight. Unbelievably, we also found Cory eggs in the bowfront. I'm baffled.

 

Water changes seem to trigger spawning.