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Thread: High pH killing Tetras?

  1. #1
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    btaylor's Avatar
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    High pH killing Tetras?

    This is going to be a long post. Please bear with me.

    I am trying to diagnose episodic, sudden fish deaths in my 25 gallon community tank. It carries mostly tetras (because they are easy-going) and has been running continuously for six years. The fish die in ones and twos without warning, preceded by no change in behaviour or signs of disease. The mortalities are not selective by age, size or species. Fish that have been in the tank for two months or two years are equally likely to succumb.

    I live in the country so we are on well water. It's moderately hard (~100 mg/L) and slightly basic. However, the well does carry iron (and possibly trace organics) which I learned the hard-way is eventually fatal to aquarium fishes. Now I treat the water with NutraFin "AquaPlus", a chelator that binds the iron and other trace metals. I do a 20% water change (a bucketful) every week unless I forget; I let the treated water sit overnight to equilibrate. I have two wells, which makes matters a bit more complex, but treating the water removed the toxicity.

    The aquarium has a bottom of non-toxic pebbles and a couple of quartz-like rocks. There are two plants, a low-growing species that gives the Plecostomus somewhere to hide covers almost half of the bottom, and a newly planted Valisneria or some such with ribbon-like leaves.

    I have never cleaned out the tank, but I did replace the three-stage filter about a year ago when it decided to start pumping water onto the livingroom floor. The newer filter seems to work well enough. I don't do chemistry often enough, but the last time I measured the ammonia was 0.2 mg/L, on the threshold of detection, and iron was undetectable, but pH was 7.5. I haven't measured nitrite, but it seems unlikely to be an issue given the ammonia concentration.

    So what's killing the fish? I have gone for months with no problem, and then suddenly they start dying off. I had a dozen tetras in the tank, four species, for most of a year but in the past two months I have lost half of them. Three more this week, on two separate days. On top of this, I have a 10 gallon tank that has never lost a fish in the past three years.

    The nature of the mortalities to me suggests an episodic upset. Possibly bouts of high pH? The literature says 7.5 is high for tetras, so perhaps the pH swings too high some days when the plants are sucking CO2 out of the water? Or the day-night oscillation is too wide? The smaller tank has no plants in it. I have started filtering the water over peat to bring down the pH a little bit, but it's too soon to know if that is effective. I broke my pH meter so I can't monitor pH till I get a new one.

    I would be grateful for any ideas or suggestions.


    Barry

  2. #2
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    jmunroe's Avatar
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    Dozens of tetras and a pleco in a 25 gallon tank with only a 20% water change a week, if you remember, and you have never cleaned the tank. This is your problem.
    A pleco of even the smallest kind in a tank that small is a huge bioload, let alone any other fish. Your nitrates are probably sky high. You probably have anaerobic pockets in the substrate from never cleaning it, so when you do start to clean it do it in small sections at a time during water changes so you don't release a bunch of gases at once and kill your fish. You need to get a gravel vac to do this. Do larger more frequent water changes and depending on the type of pleco you have you may want to rehome it or get a larger tank.
    Sounds like your under filtered as well if your seeing ammonia readings and the filter isn't large enough to process the waste. This may be fixed by lowering the bioload and better husbandry without a larger or additional filter.
    Hope this helps get you going in the right direction.
    By the way, what kind of pleco do you have and how big is it?

  3. #3
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    Thanks for the suggestions. This gives me a direction.

    A couple of clarifications. I have had one dozen, 12, tetras at the maximum, always well below the "one-inch per gallon" limit. Second, I mis-stated about cleaning. I meant I have never taken everything out and cleaned the tank. I vacuum the gravel every week when I change the water. The ammonia reading I quoted was right at the edge of the detection limit of the kit I was using. I'll check the nitrate.

    As for the Plecostomus: I'm pretty sure it's a Common Pleco, Hypostomus plecostomus. I asked for a pleco when I set up the tank to clean the algae, which it does assiduously. It's maybe four inches long. I don't believe it is anywhere near outgrowing the tank yet.

    Again, I would point out that my smaller tank, proportionately stocked, similar set-up, same water treatment regime, with a Pleco, does not kill fish.

    Barry

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    I agree with above, you should start vacuuming the gravel each time you clean your tank and that is how you should be removing your 20 - 50% of your water each week. Treat your water when you do your water changes and start testing ammonia/nitrite/nitrate (the nitrogen cycle) and don't worry so much about the pH and iron as of right now. When you perform your tests, ideally your ammonia and nitrite levels will be zero and your nitrate will be under 20, 40 being max allowed. Your nitrate is likely way over 40, which is where it become dangerous (causing your early deaths). Also clean your filter our with tap water regularly enough as nitrates will also build up in the sponge, along with the gravel.
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    Yes,sounds like old tank syndrome to me also.Change more water in larger amounts,more frequently.Your PH is not high at 7.5.Your method of changing water=80% of dirty water left over each time;over time it becomes very concentrated with TDS,oraganic compounds,even bacteria and definitely nitrates.
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    I'm with blue lagoon bigger WC are needed

  7. #7
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    Thank you, everyone.

    Barry

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