Tropical Fish Keeping - Aquarium fish care and resources

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DavidandKellyHarvey 04-11-2012 12:01 AM

Mass Fish Death
Hello all,

We are new to this forum so firstly I'd like to say hi. Secondly, I'd like to ask for help.

Recently we were given a fish tank. We decided to switch from the one we had and make this our new one. It's approximately 20 gallons. The tank we had at first was fully settled and the fish we had in it were doing great. We fully cycled the new tank before the big move and after about a week added a few more fish to increase our numbers. Everything was going swimmingly (excuse the pun).

The tank at that point consisted of:
5 Danios
8 Rasboras
2 African Dwarf Frogs
4 Black Skirt Tetras
2 Apple Snails

We are over-filtering to roughly 40 gallons.

Suddenly, in the space of just a couple of days we had mass fish death. In one week we have now lost all Rasboras and all but one Danio. There have been no signs of disease and the water chemical levels have remained pretty steady throughout (ammonia appearing briefly after some of the deaths). The strangest part of it is that nothing else in the tank has shown any signs of stress or upset. The frogs, tetras and snails have all remained perfectly happy and healthy. There are no signs of bullying from anyone and there have never been any signs of injury.

Only one physical sign appeared on one Danio and that was one small red patch on its side. I don't know if that was related in any way. The Rasboras showed very little sign of anything before death but the Danios (they held the longest) showed a lot of stress with clamped fins and drooped tails.

We took some water to PetSmart to have them test it in case our tests were incorrect but they agreed that ammonia, nitrates and nitrites were all normal. The only thing they could tell us that we didn't know was that the alkalinity was really low. They told us that this could cause a change in pH which obviously isn't good. Therefore pH has been our only lead... Wouldn't that be a killer of everything in the tank though?

One other thing was that we added a rock not long before the deaths began and weren't sure if that was somehow affecting pH. We have since removed the rock, but no changes have occurred.

The pH of the tank does seem to have changed since we first set it up. It started at about 6.8 and now sits at about 6.2. We haven't seen any change to this since we noticed that though.

Does anyone have any idea what might be happening in our tank? It's been horrible to lose this many fish as I never have before.

Any help is most appreciated. If you need any more information let me know.


Geomancer 04-11-2012 06:56 AM

Well, usually when something like this happens it is because of one of two causes. The first, and most common, is water quality. The second is disease.

For water quality, any reading of Ammonia or Nitrite is bad. There is no 'safe' level other than 0. For Nitrates, that should be kept under 40 ppm if possible. Extra filters do not really help in this. X Ammonia will produce Y Nitrite which will produce Z Nitrate. Doesn't mater how many filters you have, the amount of X, Y, and Z will be the same.

If you monitored Ammonia and Nitrite readings closely throughout this process and they were both 0, then disease may have been the problem. You say though that you tested after the deaths, and did find Ammonia. What reading did you get? And what method was the test (strips or liquid)?

When you transferred the fish, did you re-aclimate them? Likely a new tank with all new tap water will have a different pH and a different temperature. If your KH (Alkalinity) is low, then yes over time your pH will lower some. A reading of 6.2 is nothing to worry about with soft acidic water fish like what you have. However, 6.2 water is six times as acidic as 6.8 water. It is a logarithmic scale. Do you know what the pH was of your old tank?

One other thing that comes to mind is when a tank is 'cycled' it is cycled for a particular level of source Ammonia. Often, the actual amount of Ammonia produced by the fish will be different. If the fish produce less Ammonia than was used for the cycling than you will likely be okay. If the fish produce more, then you'll have a 'mini-cycle' as the bacteria will have to increase in size to compensate. This is why it is best to stock a new tank slowly, for example one school a week, to give the bacteria time to adjust. Fast growing live plants can do wonders in sucking up ammonia before the bacteria even have a chance at it.

Oh, one last thing, if your old tank was high in Nitrates, and the new tank with new water was at near zero Nitrates, oddly enough the sudden shift to clean water actually will cause a lot of stress on fish. Any changes to water quality/parameters has to be done slowly.

DavidandKellyHarvey 04-11-2012 10:38 AM

Thank you for your response.

Ammonia and Nitrite were both 0 before the deaths and have remained 0 throughout most of them. We have monitored it closely and only in the last day have we noticed a very slight increase in ammonia. The last of the dead fish was hidden behind the filter intake and I assumed it was this that caused the rise. Nitrates have stayed around 25 - 30. The tests are liquid.

When we switched the tanks we actually moved a lot of the old water as well. The fish were re-acclimated to match however.

The chemical levels in both tanks appeared to be the same. It's interesting that you mention the change in Nitrates although they were at similar levels the whole time.

Byron 04-11-2012 04:34 PM

I'll just pick up on a couple of points from what's been said to date. First, the tank is 6.8 to 6.2, what is the tap water? When testing tap water pH, put some water in a clean covered jar or container and shake it briskly for a couple minutes; this outgasses the CO2 so the pH reading will be more accurate.

Nitrates are high, anything above 20ppm can be stressful, even 10-20ppm can be to many fish; and while this in itself may not likely kill them, it stresses them and thus weakens them so other issues become more critical. Nine out of 10 fish deaths is due directly to stress from any one of many sources.

Live plants, even some simple floating plants, would help a lot in many ways. Highly recommended. Ammonia would be grabbed up pronto, and nitrates would be near zero.

To the fish. Which rasbora species? And which danio? We have profiles, second tab from the left in the blue bar across the top, and both these groups of fish are under Cyprinids. If you check the species and photos, you should be able to pin down the species you have.

The Black Widow Tetra (aka Black Skirt T) is a known fin nipper, esp in small numbers (anything less than 8 for this fish is small). This would add stress to rasbora and may be a contributor. Even if no physical contact is made, the inherent trait of the Tetra will be read chemically by other fish.

Over-filtering achieves nothing good, as Geo alluded to. There is also the issue of water current, this is stressful to fish that are forced to live in it 24/7 with no escape.

Lots of stress mentioned here, but it's a major factor in any fish death, so I hope this helps a bit. Don't hesitate to ask further, we all want to help you resolve this and be successful.:-)


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