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Tank crash - what was the cause?


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Can anyone help me ascertain why this tank suddenly crashed on me? 

This is a 54 gallon corner aquarium, dirt capped with BDBS that has been set up and running a bit over three years. It is well planted with jungle valisneria, an amazon sword, a dozen or so java fern, a floating mass of hornwort that covers about half the tank that I pull  handfuls of out every week during tank maintenence, and ever present duckweed I've never been able to get rid of. I net a fair bit of that out every week too.  It has two sponge filters and a hydor canister filter, and an eight inch airstone bar, so I can't see lack of oxygen being an issue. 

Tanks stock was originally just six cardinal tetras, a clown pleco and some spare male guppies along with ramshorn and malaysian trumpet snails, and a single nerite. For the past year it has been stocked with 17 young angelfish that I have been growing out from dime size - they were about two inches in body height at the time of the crash, and had spawned several times in the tank, although the eggs had been eaten shortly thereafter each time. The angels were being grown out in the smaller tank with the plans to move them up to my 220 gallon once they reached a good size - I had been planning on moving them at the end of August. The angels, as they grew, ate all but one of the guppies, whom they could never quite catch. These were males that hadn't made the cut for best breeding stock anyhow, so I wasn't concerned that the angels had extra protein snacks. However they left the cardinals alone.

Two days before last week's water change - I added three anubias which I had purchased online after giving them a hydrogen peroxide dip and then rinsing them. This is the only addition to this tank I have made in a year.

I do weekly water changes of 40-50%, and test every week or other week at the time of water changes as well. Tank parameters at last week's water change were

Ammonia 0 Nitrite 0 Nitrate 5 

pH 7.8 Temp 82F TDS 345 and kH 5 (no gH test, haven't got one)

These have been the normal parameters this tank has been testing at for well more than a year. Its been very stable with only the TDS ever really shifting. 

Last week two days after the water change, I noticed a dead cardinal tetra stuck in the hornwort and a spawn of angel eggs laid on one of the valisneria in the morning. I netted out the dead tetra, and tested the water parameters - Ammonia 0 Nitrite 0 Nitrate 0, pH 7.8 Temp 82 TDS 220 kH 5. I did a 20% water change anyhow. The eggs were eaten by that afternoon. 

Two days ago, I woke up to find three dead angels on the bottom of the tank. What on earth? Pulled the dead angels out and immediately began a 50% water change while testing the parameters. Ammonia .25ppm nitrite 0 nitrate 5 pH 7.8 Temp 82 TDS 300 and kH 5. Still the normal parameters.  I'm not surprised by there being some ammonia with three dead angels but why did they die? The other angels and the cardinals were still acting normally, no red gills, no flashing, no sitting on the bottom nor gasping at the top. I left to go to work - I came back and there were four more dead angels and all but one of the tetras was dead. Pulled them out, lost another two angels and the last cardinal before the night's end. I sat and watched, they were going from swimming normally to sitting on the bottom and then stopped breathing within ten minutes.   The next morning two more angels were dead. Itested the water parameters again as I did another 50% water change - still the same. In desperation I pulled the remaining angels out and threw two into a 47 column that only had some guppy fry, ottos and shrimp in it, and moved the other four to the 220 gallon - two of them passed away within the hour, I suspect system shock because the temp in that tank was only 78 and the pH in that tank due to large amounts of dirftwood is 6.8. But the other two made it. It has now been a day, and I have two remaining angels in each the 47 and the 220. I am waiting to see if they still suddenly drop dead in these other tanks.

The clown pleco and the male guppy are still in the 54, but have not died. 


What am I missing? Could the anubias have brought something in that kills this quickly? What did I screw up so I don't have this happen again? 

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Wow! So sorry. That is a tough question.

If it were people we would do autopsies. Fish detectives would ask who died and who didn't and what happened in the hours before death.

So the pleco and the guppies didn't die? All the deaths were angels and cardinals?

No red gills? How about any other redness, any red blotches, any red streaks in the fins, however slight?

Despite you having ruled out water parameters through testing (good work), common sense would note some of the fish that were moved revived and continue to live. This paradoxically points back to water parameters. We don't know what it is, but being moved away from the death tank was the key to not dying. So it doesn't seem like a 'disease'.

Last time I had sometime similar happen, it was me putting rotting food in the tank. I believe it was an ammonia spike (but testing show a little, but not fatal amount of ammonia). With my incident, I think the plants had absorbed the worst of the ammonia by the time I measured. In my case, a near 100% water change revived a few while a few other fish continued to perish. The remaining fish recovered completely.

It will be interesting so see what others say.

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The guppy and the pleco are still in the tank, both are alive and fine. No red gills on the guppy. I can't honestly see on the pleco, he's a shy little thing. But he's still scooting around behind the driftwood in the back of the tank so I gotta figure he's fine. It was just the cardinal tetras and the angelfish.

I did not see any red gills on the angels or the cardinals. Which you would think if they were in distress they would have. I did look at them after they died, they did not have red gills. I also did not see any open wounds or missing/raised scales or swollen eyes or lumps or split/red streak fins. Now, the caveat on the fins is that most of the angels were black angels so I couldn't  see any color changes in the fins. So, if there were blood streaks, I couldn't see them. Had a couple lighter blue marbles, I did not see any red streaks on any of their fins. I was expecting to find at least some physical damage to a few angels given there has been breeding activity in the tank by several pairs, which try (and fail) to chase off other fish from their eggs. But I didn't even see any physical damage, which I know angels are well capable of doing to each other, as I've had other angels rip pieces out of each other in fights.

I am perplexed by this. It seems like it should be a lack of oxygen or some other parameter issue that would kill this quickly. But I can't identify what, if any, parameters are incorrect. But if its an incorrect parameter, wouldn't that have also killed the guppy and the pleco?

This tank has been fed Hikari vibra bites, from a bag that is less than three months old, frozen blood worms, frozen mysis shrimp (both packets less than three weeks old) fluval bug bites (about a month old) and Hikari algae wafers (for the pleco, bag less than four months old). So I don't think the food could be a culprit, especially given that all of those foods are fed to various other tanks and I have experienced no losses in any other tanks.

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@Nataku I feel this is an excellent post.   It's a real world problem, with many possibilities.  I have been thinking about this situation since you posted.

Testing water parameters, that many fish-keepers test for, are very important, but often times are only a small glimpse of what is really going on, IMO.

Seems like the problem started shortly after the water change and only got worse with more water changes.


I have a few questions just for my own curiosity:

Are you on a well or city (tap) water?

Do you use water conditioner?  

Did any other tanks have any water changes, near and/or around the time of this particular issue?

Do you have an air-stone in this tank?


We may or may not ever know the real reason, but I feel it helps to talk about it.  Who knows, one of us may come up with the correct question and/or answer, yet.

All the best......


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I see the angle you are going with, and I thought about that as well! 

I am on city water, and I am in Florida, so the city water is drawn from the aquifer and treated with chloramine.

I use Prime as a dechlorinator. Have for years. 

I did test the water straight from the tap as well after the 50% water change after the first set of dead angels, wondering if I could be exacerbating the issue somehow. The water from the tap tests out at pH 7.8 ammonia 0 nitrite 0 nitrate 5 TDS 190 kH 5.

I did water changes on four of my seven tanks that day, and did the remaining three the next day. None of the other tanks have experienced any deaths. 

The 54 runs two sponge filters and has an 8 inch airstone bar as well. They all run off one air pump with a splitter to run lines to each. Tank is also well planted, so I don't  think even with the tank at 82 degrees, that it could be insufficient oxygen. I do not have cO2 on any tanks, so I can't  have overdosed too much of that into my tank.

Sadly I am doubtful I will ever know exactly what happened to this tank, although if we can put our collective heads together and figure it out that would be great. I just don't  want this to ever happen again, but its hard to prevent something from happening when I don't  know how it happened. 

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Sorry to hear about your angelfish and cardinals. 😞 The only time something like this happened to me was when I accidentally forgot to dose dechlorinator in one of my tanks. Chlorine isn't something that most experienced fish keepers test for, so it's easy to miss. Two days later, fish started dying. My husband was the one who figured out my mistake, so I did a large water change with dechlorinator added. However, the fishes' gills were already too burned by then, and they died en masse in the following days. A few fish survived though, which was a relief.

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I suspect it was something to do with being a dirted tank. @Cory was recently talking about his own bad experiences with a dirted tank causing tank crashes. There are a lot of factors in play with all that bio-mass in the tank. What kind of test kit are you using, strips or liquid? 

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@MickS77I use the API liquid drop tests, HDE infrared thermometer and an HM digital TDS sensor.

I have also wondered if it could be because its dirted. Its currently the only one of my seven tanks that is dirted. The other two dirt tanks I used to run have long since been broken down and given away to friends who wanted a tank or wanted the entire stocking I had in the tank. This 54 tank that crashed was started January 12, 2017. I had always assumed that when dirt tanks started to 'run out' of nutrients that the plants would slow down in growth and start to show nutrient deficiencies and then die. The plants were still growing great in there, valisneria spreading all over and still having to be trimmed regularly  (once it gets over about four feet long in that tank it got trimmed) and still pull hornwort out by the handful weekly. I wasn't aware that dirt tanks could release anything so toxic to fish this far into the established game? Aside nitrogen bubbles but its a dirted, capped tank - I don't gravel vac it so I don't stir that up if its in there.

The guppy and the pleco are still in there, still alive and active. I see no red gills or abnormal behavior from them. I know plecos can be bulletproof but that little male guppy? I assumed whatever it was would have gotten them too.

The guppy and pleco will be moving soon. I'll  be breaking this tank down shortly and letting it and all its equipment get cleaned and sit dry for a month before I restart it. Maybe longer.

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  • 2 weeks later...

the only thing i can think of after reading everything is that planting the new plants uncapped a bit of the soil and released something very deadly that does not give effects to a fish that are easy to see. guppy's in a lot of cases can be more hardy then cardinel tetras and plecos as you said can be bullet proof.

yet this is the only thing i can think of on the spot i have no idea what could have been released from the soil. this is just my opinion of what went wrong

this is my input to this interesting mystery. 

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  • 2 weeks later...

These two ideas are just stabs in the dark.

1. Did you attach the new anubias to something? If so, did you use superglue? If so, was it gel superglue? I understand that the non-gel superglue can be toxic, but I don't know in what way or how long it would take to hurt any fish.

2. I had a horrible experience with overly enthusiastically spot treating hydrogen peroxide on BBA on submerged plants (which means it was introduced directly into the water, even if aimed at specific areas). All seemed okay until the next morning when I found four dead pygmy cories, three dead nerite snails, and one dead peacock gudgeon. I lost the other gudgeon by the end of the day. Unaffected in the tank were four juvenile and three adult guppies.

Just ruling out stuff.

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