Unexplained Fish Loss

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tackful

AC Members
Mar 15, 2007
604
9
18
Sausalito, CA
Hello All,
Hoping someone can help me with a big problem. After 10+ years of fishkeeping, I have now lost 7 of the 14 tetras that had formerly been living happily in my 29 gal planted tank for years. I do a weekly 50% WC and gravel vacuuming and clean out the cannister filter every other week. This has been the routine forever,
The only change that seems to coincide with this die-off is changing the filter outflow from a long spraybar to a simple swivel hood attachment. In both instances the outflow is aimed directly at the co2 diffuser, sending the bubble down and out on a long, merry ride through the water. Plants have been happy and growing vigorously.
Am trying to figure out if the changed outflow may have somehow increased the co2 concentration to unhealthy levels. One evening I did find them breathing at the surface in a corner, but I have two reasons for not confidently determining excess co2 to be the cause:
1) Unlike the handful of times in the past when this had occurred, turning off the co2 and taking steps to maximize surface agitation did not change their behavior within a short time.
2) The die-offs had started several days before.
I am presently wondering if the changed outflow caused a slight co2 buildup to the point that the fish were being slightly stressed all along and had finally had enough, eventually needing to headi to the suirface
Thanks for reading, and all help appreciated.
 

Wyomingite

Fish Wrangler
Oct 16, 2008
525
280
72
52
Wonderful Windy Wyoming
Real Name
Ivan
The nitrate levels aren't too concerning. Nitrates aren't immediately toxic, but rather result in long term health effects from continually high levels. In addition, susceptibility to nitrate poisoning varies by genus and even species. Long term, anything over 20 ppm will have adverse health effects, with the severity of effects increasing and onset of symptoms occurring faster the higher the concentration. Short term, however, most of the species we keep in our tanks can tolerate levels of even 40-80 ppm for a few days or even a couple of weeks without any long term issues, again based on species and genus. Trout and many other fish from fast-flowing, clean and colder waters start exhibiting adverse effects or even death at or above 5 ppm in a matter of days, for example. On the other hand, studies on striped bass in recirculating culture systems show that they can handle concentrations at 200 ppm and not start exhibiting adverse effects for a week or so, after which a rapid decline in health occurs. Fish that have been kept in a tank that has never had a water change can adapt to levels over 200 ppm and do not exhibit any outward sign of ill-health, but the health issues are there: stunted growth, poor healing and a compromised immune system, sterility, and more. Sudden water changes will kill these fish because their body can't handle the sudden shock from the change in water chemistry. I've rescued fish that maxed out test kits, but I don't remember the upper range of those kits. Size is also a critical factor, with larger fish typically being able to handle higher levels than smaller ones. Incidentally, the stunted growth of fish kept in high nitrate concentrations is one part of the myth that fish only grow to the size of the tank, the other part being that they often die before achieving adulthood due to poor water conditions and health problems.

As long as you are keeping your nitrate levels below 20 ppm long-term, a few days at higher concentrations shouldn't be an issue.

I wouldn't think changing the type and direction of the outflow would cause a die-off like this. A couple of things come to mind, and it might be a combination of several factors. If it hadn't been half your fish all at once I'd say it was likely age, as most tetras don't typically live for more than 3 or 4 years anyways. The same with a toxic contaminant, since it only was half. I have to wonder if these fish had a negative reaction to something that wouldn't normally cause harm, but did to them due to age. Can you think of any other changes, even minor, that could have contributed? Can you think of anything that might have been different in your maintenance regimen, no matter how small?

WYite
 

tackful

AC Members
Mar 15, 2007
604
9
18
Sausalito, CA
Thanks Wyomingite. Based on your information, I will cut down on my nitrates. I know that my N03 dosing is well above EI recommendation for a 29 gal tank. Regarding other changes, there are two small things that I've wondered about. First the dog got skunked right around the time the die-offs began. I washed her thoroughly with the appropriate solution then burned a lot of incense in the house for the next few days. The other matter involves my co2 diffuser, which I have been cleaning with bleach or Tilex for years, followed by a thorough air flush and rinse. About three weeks before all this, I ran out of both mixtures so used a pinch of Comet powder instead.
Fishorama, I'm sure that I didn't miss on the Prime during WC. I don't have information on a water co. "flush", other than to say that they are very reputable and my LFS hasn't had any problems.
Loach, unfortunately didn't get those readings, but plan to do so this evening after the co2 has been going for 5-6 hrs. Incidentally, co2 has been turned down as a precaution.
 
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