Cory habrosus died today; some others seem listless or swim/lie oddly.

Hello! I found one of my eight C. habrosus dead on the substrate at the back of my tank today. About half lie on their sides frequently or swim oddly. Got them 8 months ago; they were the first fish I put in after cycling. When the strange behavior first happened a couple months ago, I removed a few to a hospital tank and dosed antibiotics. They eventually recovered enough to put back in main, and the one that seemed at death's door is now doing fine. 29g planted main tank. I keep ammonia and nitrite at 0 and nitrates under 20. pH stable at 7.5. Tankmates betta, 3 ADFs, 9 cardinal tetras, 4 amano shrimp, 6 adult assassin snails plus at least 4 assassin babies born in-tank. Cories get bottom-feeder pellets and Hikari sinking wafers. Some white (mold?) patches on driftwood and java moss - might these be toxic? If so, why would some previously sick be recovering? Too, the females are big and lazy but have never been ill. Thanks for any help you can offer!

PS. Tank temp. kept at 78F; daily 5-10% water changes w/Prime & alkaline buffer to keep kH up at 3.

Pictures and video of the fish?

It looks like I can't upload the mp4s, but here are a few photos. They do have a sand dish where they can eat and rest if the gravel begins to irritate their barbels.

Colin will have a better idea of specific disease here, but I have noticed a couple of serious issues that are not going to help the fish.

First, do not use alkaline buffer. Corydoras, particularly the small species like C. habrosus, are sensitive to any substances added to the water. There shold be no need for an alkaline buffer anyway, let the pH do what it naturally wants and it will stabilize and remain more constant long-term which is better for the fish. You don't give the GH which is the more important parameter, but if the pH lowers without the buffer I assume the GH is on the soft side, which is good. An acidic pH will be good too, as far as mentioned fish go.

A betta, especially if a male, is not a community fish, and may be an issue whether you see something or not. Not likely to be the culprit here, but could be a contributor if nothing more.

Whit mold...is this what is showing in the third photo? Mold/fungus from wood can be deadly toxic, and while this will generally affect cories first, it may or may not be the issue. Not all fungus is toxic, but some definitely is, I had it myself and have known others who have as well. Cories were the first fish in all cases to show symptoms leading to death. Something to be aware of. What type is the wood? This seems much more prevalent on branchy wood like graperoot/grapewood and similar.

The gravel is a problem for these fish. A sand dish is not really the answer, as cories browse all over the substrate, and they "expect" to take up mouthfuls of sand and expel the sand through the gills; not possible with gravel, regardless of sharpness or not.

After 8 months, cycling is not at all likely the issue, and your ammonia/nitrite at zero is good. Nitrates close to 20 ppm is high, is there a reason you cannot get them lower?

My kH falls to 1 w/o alkaline buffer. The lfs also adds this bc local water is so carbonate-soft. gH stays btw. 6-7 at all times, which is where I like it; any lower would be bad for the shrimp. Vern the betta is shockingly mild-mannered, and the cories will swim right up by him. I worry more about the ADFs, but the cories show no fear of them, either.

The spots on the log in the 3rd photo are what I was talking about. It's mopani driftwood. Is there any way to conclusively determine if it's toxic? Or any way to generally sanitize it w/o causing other health issues?

Should I add more sand? I understand it tends to fall below gravel, anyway, though. My main concern was just that they have the option to be on sand if desired. They usually only go there at night, though.

I usually keep nitrates around 10; 20's as high as I ever let them get before intervening, not the typical value.

I'd be happy to use something that increases kH without pulling pH up to 7.5; just not sure that exists. I don't want an acid crash if kH is exhausted, though.

JoshOfMichigan said:

My kH falls to 1 w/o alkaline buffer. The lfs also adds this bc local water is so carbonate-soft. gH stays btw. 6-7 at all times, which is where I like it;

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My KH is zero and has been for over 20 years. Most soft water fish are better with an acidic pH, so you have to balance their health with the shrimp (if they are why the pH is "kept" at 7.5). The GH at 6-7 presumably is dH not ppm (?). The C. habrosus and the cardinals would thrive in a zero GH/KH with an acidic pH in the 4-5-6 range. My earlier comment stands, the cories and cardinals (characins are also highly sensitive to chemicals) will be better for not using additives. I realize I am talking fish here, but that is what really matters, their health.

I forgot to mention previously that the frogs should not be kept with fish. Other members like @Essjay have tried this and can explain the problems.

JoshOfMichigan said:

I usually keep nitrates around 10; 20's as high as I ever let them get before intervening, not the typical value.

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This is a problem. Nitrate should remain constant over years. If it rises from one water change to the next, something is wrong. [I assume nitrate is not entering with the fresh tap water, but accumulating within the aquarium biosystem.] The usual causes are insufficient water changes, insufficient filter rinsing, overcrowding, and/or overfeeding. Live plants, esp floating like Water Sprite, Frogbit, etc, can make a difference too not by using nitrate but by using ammonia/ammonium faster than the bacteria so nitrite then nitrate is significantly less. As close to zero is the aim. A water change once a week should be sufficient, but greater volume; my tanks used to get a 60-70% change every week, now they are messing with the water and I have had to reduce this back to 50%, but provided parameters of tap water and tank water are reasonably the same, the more water changed at one time the better for the fish and the system.

JoshOfMichigan said:

Should I add more sand? I understand it tends to fall below gravel, anyway, though. My main concern was just that they have the option to be on sand if desired. They usually only go there at night, though.

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The substrate should not be a mix, but just smooth sand. Play Sand works well, if quality; in NA the Quikrete Play Sand available from Home Depot or Lowes is about as good as it gets, unless you use the much more expensive aquarium river sands. The gravel is impacting the cories. It is also dangerous due to food than gets down where they cannot reach it, and this plus other organic matter can cause serious bacterial issues. Gravel with cories is quite frequently a bacterial problem more than the rough/smooth issue.

OK, I've removed the mopani and scrubbed it down with a toothbrush under hot bathwater; I now have it soaking in a bucket of Prime-treated water to bind any chlorine compounds. I've also removed any other fungus I can see, mostly by removing java moss and combing through it. I'll also do a good thorough gravel vac of the area under the driftwood.

I've recently added water lettuce and started allowing the duckweed to do its thing so as to help with nitrates. A little of the nitrate actually is coming from the tap water.

I've heard mixed messages about kH and gravel/sand. There are scuds which take care of what gets between the gravel. But I do try to do two gravel vacs per week to minimize bacteria. If the cories don't start improving after a few days, and I can get some confirmation from other experts that it's OK, I'll taper off the buffer and let kH fall (if I understand you correctly, none of these animals need any kH? I want to be darn sure of that before making a change.) You're the only person I've seen say that 0 kH is acceptable. I've seen others say to keep it at 5, but that's more buffer than I need. I also don't want to let pH fall below 6.5, especially not quickly. Shelled animals. Again, the buffer is *not* for keeping pH that high; that's an undesired side effect of trying to keep any kH at all. I'd honestly prefer a neutral 7.

Switching entirely to sand would be an extremely disruptive process that I'm only inclined to do if all else fails. Among many other things, I try not to mess with the betta territory too much to keep Vern as chill as he is.

Also, my understanding is that such acidic water as you'ryou're recommending is only necessary for characin reproduction, not adult general health.

First off here, I want to say that I am not trying to be argumentative, my advice is intended solely for the well-being of the fish. Sometimes I come across as arguing for the sake of arguing, but that is not the thing--there is considerable false information being dispersed in his hobby, some well meaning no doubt, but lacking the knowledge to be useful.

JoshOfMichigan said:

(if I understand you correctly, none of these animals need any kH? I want to be darn sure of that before making a change.) You're the only person I've seen say that 0 kH is acceptable. I've seen others say to keep it at 5, but that's more buffer than I need. I also don't want to let pH fall below 6.5, especially not quickly. Shelled animals. Again, the buffer is *not* for keeping pH that high; that's an undesired side effect of trying to keep any kH at all. I'd honestly prefer a neutral 7.

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This myth about KH is relevant for aquarists who keep livebearers and other species that absolutely require harder water and (usually) a basic (above 7) pH. It does not apply to very soft and soft water fish species. Consider that these fish have evolved to "live" in water than has zero hardness, no buffering, and an acidic pH. The Rio Negro has a pH in the 4's, and it is full of fish like cardinal tetras, pencilfish, hatchetfish. I have made fish habitats my primary concern over the past decade-plus, and I understand how absolutely critical these things are to the health of the fish. The relationship of a freshwater fish to its environment is way more crucial to the life of the fish than that of any terrestrial animal. Substances in the water that are able to diffuse across the cell membranes enter the fish with the water that is continually passing through the cells via osmosis. The fish then has to somehow deal with these, and the farther things are from the expected habitat conditions the more it drains the fish of energy needed for other functions.

I am lucky to have tap water with zero GH and KH, and a natural pH around 5. They are now adding soda ash to raise the pH, but it is temporary and has no effect at all on GH or KH. My tanks are full of soft water fish. My cories spawn continually (other fish eat most of the eggs, that doesn't matter), and so do most of the other species. The fish have evolved to function best in such an environment, and while "adaptation" is possible for some species, the fact remains that there will always be less problems for the fish if they are living in what their physiology is designed for. Ian Fuller is an authority on Corydoras, perhaps the highest living today, and he keeps his wild caught cories in zero GH/KH and an acidic pH. The fish's genetic makeup expects this.

There is no such thing as a pH of 7 in natural habitats of any freshwater species. Water is a powerful solvent, and as the "pure" condensed rain falls it picks up minerals, organics, etc, and the habitat water is either on the acidic side or basic side. This impacts the life processes of fish, since they are "built" for a very specific environment, though the GH is directly of more significance here than the pH provided the pH does not fluctuate. With a given GH/KH the pH will settle at some value and be quite stable, though interference by the aquarist can wreck everything.

I am talking fish here, again, not shrimp of which I have no experience. But when any of us decides to maintain "x" species, be it fish or shrimp, we have an obligation to provide the best environment. And sometimes, often perhaps, there is no real middle road.

JoshOfMichigan said:

Also, my understanding is that such acidic water as you'ryou're recommending is only necessary for characin reproduction, not adult general health.

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As my previous post indicates, this is another false assumption in this hobby. The closer the water is to the habitat parameters, the healthier the fish will be. Other factors of the environment also matter...light, current, substrate, rock/wood, and of course other fish species. Stress occurs when any of this is significantly out, and stress is the direct reason for 90% of all disease in aquarium fish.

I understand you're sharing your knowledge and experience, and that tone doesn't come through over type. No worries.

I gather you specialize in Amazon River Basin fish? It's exactly the shrimp and snails that I worry about if I let kH fall to 0 and pH below 6.5. There's a website I used to determine compatibilities and stocking levels, well-known but the name escapes me right now. It recommended pH of 7-7.5. Neutrality may not be natural, but it's also not actively offensive to most animals, so far as I can tell. My tetras are thriving; plump and active. Oh, I should also mention that the cories are likely tank-raised, not wild-caught. They've even - when healthy - gone through their spawning rituals in my tank, though of course they won't actually reproduce in the parameters described. The lfs breeds most of its own fish, though I'm pretty sure the tetras are an exception.

Worst-case scenario, my hospital tank does have an all-sand substrate, but it's only 5 gallons and I want to make sure its little heater can handle keeping temperatures stable through a Michigan winter before transferring anyone over. If nothing else improves the cories' health, and I can keep the temp. in the 70s on polar vortex days, I'll do that.

Byron said:

This is a problem. Nitrate should remain constant over years. If it rises from one water change to the next, something is wrong.

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There might be a bit of a confusion with the statement above. If you measure before the change the water it should be the same as the previous time just before the water change and the same the next time just before the water change. Assuming no nitrate in the tap water if you do a 50% water change you would expect the concentration of nitrate immediately after the water change to only be at 50% the concentration before the water change. I don't know if most people are regimented enough to measure the nitrate at the same point in the water change cycle.

Uberhoust said:

There might be a bit of a confusion with the statement above. If you measure before the change the water it should be the same as the previous time just before the water change and the same the next time just before the water change. Assuming no nitrate in the tap water if you do a 50% water change you would expect the concentration of nitrate immediately after the water change to only be at 50% the concentration before the water change. I don't know if most people are regimented enough to measure the nitrate at the same point in the water change cycle.

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If I am reading this correctly, I don't think it is saying what I intended. So I'll explain what I intended, lol.

Test nitrate before a water change and it is say 0 to 5 ppm. [My API liquid test measures in fives, so it will show 0-5 ppm, then 5-10 ppm, then 10-something (can't remember, it never gets up this high).] After the water change, it should still read 0-5 ppm. And never rise above this, ever, no matter how long between water changes. My tanks have all run in the 0-5 ppm range for over 10 years. I initially tested every week (before and after the W/C, then same the next W/C, etc), then I went to sporadic tests maybe once or twice a month, then once a month for a couple years, then...none. I know by then/now that the biological system is established. Now, I know this was the case with water changes of 50-60% once every week without exception; I've no idea if missing a W/C would have produced a change. I rather doubt it would, with the plant load and balanced fish load and my minimal feeding, but I can't be certain. For ten years I do know the nitrate never rose above 0-5 ppm, and may well have been zero all that time, or it may have actually been say 4 or 5 ppm before the W/C and zero after, this tet is not suited to that level of exactness. But when we are talking about such a minimal rise in nitrate at the very most, there is nothing to worry about.

I did have one tank that suddenly tested in the 5-10 ppm range, for months. It had some sort of organic cloudiness that I could not resolve. Neale Monks looked into this with me, and we could not find anything that might be behind this, and it lasted for close to three years. At that point I tore the tank down because I was moving and downsizing tanks anyway. I dumped the sand out just so I didn't carry the issue to another tank, and I got rid of the wood. Neale and I thought it might have been something about the wood, there was a lot of it, and this can happen.