mysterious oxygen deprivation

I posted a while back about how my healthy minnow tank "maxed out" on O2 one day and since then has been able to support fewer and fewer fish no matter what kind of equipment I use. Now I'm down to just one minnow gulping at the surface for air, in a 30G with powerhead and a supplemental aerator hooked up to the intake.
I was suprised when common shiners added from the creek with plenty of acclimation time showed classic signs of oxygen deprivation within a day or so, considering I had a whole school of them just a few months earlier. Only the 3 golden shiners I got from a bait shop survived, and now their behavior has changed from a happy roaming of the tank to their noses constantly at the surface day and night as if they can't get enough air. The fact that long-term captives are exhibiting the same behavior as new additions within their first couple of days eliminates a lot of strictly "new fish" or "eventual" culprits. Now the ones with the lowest O2 tolerance are dropping in order from smallest to biggest as they would in a tank with no air at all.
Although I got some good suggestions before (like the electricity in the house might be low), people tend to go off in different directions that aren't necessarily wrong but starts a debate that never returns to the subject of the post. Let me ask a simple question, if you had a large tank with a visibly working aeration system putting out plenty of bubbles, and you had proof (either by autopsy or O2 meter) that a single fish was having oxygen issues, hypothetically how is that possible? And if you didn't know, what else could it be? Here's a pic of my very ordinary setup.

Might I suggest testing the water and sharing the ppm or mg/L results with us? You can buy a general test kit that will measure things like ammonia, nitrite, nitrate, pH, and DH, and then a specialty test kit to measure dissolved oxygen.

Might I suggest testing the water and sharing the ppm or mg/L results with us? You can buy a general test kit that will measure things like ammonia, nitrite, nitrate, pH, and DH, and then a specialty test kit to measure dissolved oxygen.

Nitrite in particular will cause fish to act oxygen deprived in water which is well oxygenated. It acts on fish in a similar manner to how cyanide acts on people, blocking the oxygen receptors on the red blood cells. But for nitrite to persist for more than a couple weeks even in a new tank is pretty rare. The only likely scenario I can see would be one where nitrite levels rose rapidly while pH crashed, and not enough water changes have been performed to get the pH up into at least the mid 6s so nitrification could resume. But this is all speculation without numbers.

Edited by Subrosa, 09 February 2013 - 02:50 PM.

What type of filter are you using?

It's "hypothetically possible" if the air above the water is lacking oxygen, but I'm guessing that's NOT the case and something else (chemical? bacteria? protozoa?) is more likely poisoning the fish or damaging their gills and causing suffocation. Could the air pump be drawing in some toxic fumes from something nearby?

Is the water much warmer then their natural habitat?? could the water in your aquarium hold less oxygen because its warmer then their native habitat???

Smaller tanks are more prone to go off-cycle where biofiltration fails. Even with no supplemental aeration, oxygen levels should be adequate assuming feeding is not excessive. I agree with others, some toxin / metabolite is involved that interferes with respiration.

Checking ammonium, nitrite, and nitrate levels are all excellent suggestions. I'm suspicious that the pH may have gone too acidic or alkaline. Most freshwaer fish prefer a pH between 6.5 and 8.0 with some species preferring the higher or lower areas of that range. Some fish do better in farther extremes.

If I had your tank, I would check the pH and then do a massive water change, probably about 80% new water. I would switch filters, using one from a healthy tank to keep useful bacteria, or at least clean the filter media. Make sure that the media does not dry out and kill beneficial bacteria. I would remove all of the rocks and other materials from the tank, except plants.

According to apparently reliable internet sources, high pH can damage gills, low pH can cause thickening of gills. Either pH extreme can cause release of toxic compounds from the rocks or other added materials in the tank, which may damage the fishes' gills. Damage to gills will cause the fish to gasp for air. I suggest checking the gills on fish that have recently died or are showing symptoms by gently and slightly prying back the operculum. Compare them to healthy, live fish of the same type. If the gills are not as long and feathery, whitish, brownish, or other color from the healthy fishes' gills, you probably know why they are gasping.

Phil Nixon
Tolono, IL

Thank you all. When I took a water sample to the pet shop and was told "we can't test the O2 but I can test the PH" I thought "why would I test PH for an oxygen issue?". I didn't know that nitrites caused breathing problems, I guess because I've never seen it.

Thank you all. When I took a water sample to the pet shop and was told "we can't test the O2 but I can test the PH" I thought "why would I test PH for an oxygen issue?". I didn't know that nitrites caused breathing problems, I guess because I've never seen it.

Sounds like a low effort pet store. It might be time to buy your own test kits. They are inexpensive online; amazon, ebay, and pet websites sell them. The Tetra Test Laborett is a good general kit that is $20 at most pet stores and personally I use the Jungle 6 in 1 test strips, $8 at pet supplies plus. It looks like dissolved oxygen test kits will run $20 to $30 online.

Edited by EricaWieser, 21 February 2013 - 08:07 PM.

Not to throw stones at critters living in glass houses, but I was wondering what kind of rock that is with the drilled hole through it. I can't see it well enough to tell. But not all rocks are inert.

Dissolved oxygen needs to be measured on-site, preferably in the tank. Scooping water out of a tank and pouring it in a bag will change it almost instantly. Even if you submerge a bag in the tank and seal it up with no air space, algae and bacteria in the water can substantially change it while you're driving to the store. It would be pointless for pet shops to test for dissolved O2 in water samples that customers bring in. Besides, an air pump is cheaper than an O2 test kit, so why bother?

an air pump is cheaper than an O2 test kit, so why bother?

Because it's most likely not the O2 that's the problem but only testing it and getting a normal level will officially rule it out.