Well it's been a while but my bottom dwellers are dying...

Slowly, one by one. No signs of parasite or infection. No new fish have been added to this tank in over 3 months. No new anything, really...

Water parameters all check out. Nitrates, nitrites, ammonia are within tolerance, GH and KH haven't changed in months. Smell test is fine. Pleco, Clown Loaches, and column swimmers are just fine. The Cory Cats, though, are just dying. Every day I wake up to another dead Cory. I religiously change the water.

So what is causing the Corys, but nothing else, to perish? I thought maybe, somehow, the sand in the tank got toxified (is that a word?) and since the Corys are always on the sand, they're going to be impacted and may die.

I should have listened to my friends who said sand is bad and just do gravel. But here I am so, where do I go from here?
Current stock
36" 50 gallon
1 pleco
2 clown loach (small, young)
8 cardinal tetra
6 harlequin raspbora
2 panda cory (others died)
1 black neon (got caught in the net when I got the loaches so I said what the heck keep him, and he's been doing great)'

By my measurement this is an understocked tank. Where to go from here?

It would help to know the exact numbers for ammonia, nitrite and nitrate (I have no idea what "within tolerance" means). Also, the GH and KH. And pH. And the temperature.

How often are water changes, and what volume?

Sand in itself is not a problem, I changed all my tanks to sand 6 years ago.

Clown loaches cannot remain in this small a tank, and they need a group of five minimum. These are highly social fish, and they will grow to a foot and need sufficient space to do this. So unless you have a 6-foot minimum tank, I would re-home the clowns. If the pleco is a common pleco, it too needs re-homing as it will get to 18 inches, and it has a significant impact on the biology as it does.

Cories also need a group, at least five but more is better. But you are right to sort out the problem before getting more. The numbers asked for above may help us with this.

Byron,

I did a full chemical panel today including my fancy new SeaChem Free + Total ammonia test kit.

Nitrates: 20~ ppm
Nitrites: 0 ppm
pH: ~6.2
GH: 50-100 ppm
KH: 100-200 ppm
TOTAL ammonia: 1.0 mg/L
FREE ammonia: 0.0 mg/L
Reference sample of 1.0 mg/L total ammonia: tested at 1.0 mg/L
Water changes are weekly, every Sunday, I change 5 gallons out of 50 including a light substrate vacuuming for the pleco waste which is prodigious in amount lol.
I use a BioWheel 400 that takes Rite-Size "E" cartridges which I change monthly as recommended. I inspected the biowheels, they are turning well, all 8 nozzles are functioning.

The water is crystal clear and doesn't smell "off" at all.

Free ammonia is NH3, the toxic form. There is also ammonium, NH4, which is basically harmless. Total ammonia is both together. So your tests indicate 1 ppm (ppm = mg/l) of ammonium. This is because the pH is acidic at 6.2, and in acidic water NH3 changes to NH4 automatically. That is a simplistic explanation, but all you and I need to know. So no problems there. The plants will take up the ammonium freely, as will Nitrosomonas bacteria, same as ammonia. If you have live plants, seeing any ammonia/ammonium is odd; without live plants, it is likely resulting from the pleco and water change issue I will come to.

Nitrate at 20 ppm is the highest you want it. It will be better lower, as low as possible is the goal. While not as rapidly toxic like its cousins ammonia (NH3) or nitrite, it is still toxic to fish. The higher the level, and/or the longer the fish are exposed to nitrate, the more likely it will affect them. How it does it not fully understood as this is an area that was considered "safe" for so long. Now, have you tested the source water for nitrate (and ammonia/ammonium while you're at it)? If the nitrate is occurring partly or completely in the source water, that is one issue; but if totally within the tank, that is another and more easily dealt with.

Assuming your response will be tank-source ammonia/ammonium, you should increase the volume of the partial water changes. A change of roughly 10% (5 gallons of 50, if "50" is the actual water volume accounting for substrate, etc) is almost useless; not quite, but very minimal benefit. Closer to 50-60% is what you want to be doing. This should lower the nitrate significantly, again assuming it is tank-originating--and the pleco waste is a likely a major part of this, so keep that minimal.

Other numbers are fine for soft water fish. The filter I am not familiar with; can you explain just what these cartridges contain and are supposed to do? If there is some sort of chemical filtration for example. Are there any live plants?

I realize this is not likely getting us to the issue killing the cories, though I will say that they are very sensitive fish, and pandas in spite of being commercially raised now are still one of the most sensitive species. The nitrate is not going to be favourable to them, nor the pleco waste they are digging into, but one has to look at all factors and remedy where one can to improve things. It is always possible the clown loaches are pestering the cories; loaches are territorial, some very much so. And being too few, their aggression is going to be increased.

Byron.

I tested my source water, both de-gassed or not (because I know you'll ask!) and came up with relatively the same answer. The nitrate source is tank-based.

The tank is what I'd consider medium-dense planted, here is a picture I took tonight (color is funny b/c it's in "sunset" mode for 20 mins before the light goes out):


Here is what the filter looks like:
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=13037

It has both physical (floss), chemical (activated carbon) and biological (the wheel).

https://www.amazon.com/dp/B000HHOI8G/?tag=ff0d01-20 is what the filter assembly looks like

As for the larger fish that will inevitably need re-homing, my hope is that in due time, I will have a suitably large (100~ gallon) tank for them to finish growing up in.

Hi

Water changes are weekly, every Sunday, I change 5 gallons out of 50

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In my opinion a 5 gallon water change on a 50 gallon tank is not enough, I would be aiming at 50% water change, Now I know some people will say things like " but that's 25 gallons " and my reply is " Yes I know ", I have a 100 gallon tank and I change 50% of the water once a week with temperature matched treated water, I use a plastic 44 gallon drum that I fill the day before stick a heater in it....... You get the idea.

A few things from the text and photo.

Plants. I will assume you are not using any fertilizer, so one complete liquid like Seachem's Flourish Comprehensive Supplement for the Planted Aquarium or Brightwell Aquatics' FlorinMulti would help. Whichever, get exactly these products as booth manufacturers make several different products in their respective lines. For the swords, Flourish Tabs would help a lot. Is that Java Fern along the front? If so, make sure the rhizome is not buried or it may rot.

Nitrate. This should be closer to 5 ppm or even zero here, so increase the water change to 60% weekly (at one go, which is better than less more often). That will help the fish.

Filter. I would not use any chemical filtration, including carbon pads. As it says on those links, they trap organics, and organics are prime plant food. Might as well use the natural plant food. You can replace the chemical pads wiith plain foam/sponge/floss cut to fit, or the ones made for the filter that are just plain. Rinse them out weekly, replace if they get tattered to the point they are no longer effective.

Aquatony said:

As for the larger fish that will inevitably need re-homing, my hope is that in due time, I will have a suitably large (100~ gallon) tank for them to finish growing up in.

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This is a serious problem. First, you must have a group from the start; once fish begin developing they need what is programmed into their DNA, and without this they will not develop properly, physically, mentally, behaviourially (if that is a word). [See Loiselle's comment in green in my signature block.] Loaches need at least five of their species to develop the essential social interactions. Five would work here for a few months. Which brings me to the possible larger tank...fine if it happens, but what about if it doesn't? I learned a maxim several years ago, that I will never acquire any fish for which I do not today have a suitable tank for it at maturity. Nothing less is fair to the fish.

Well, several issues resolved themselves (pleco, clown loaches are DOA). I am down to the same 6 cardinals, 6 harlequins and 1 lone panda catfish.

I treated with Melafix for the last week and the fish look good on the outside, anyway. No signs of parasite or fungus. They're eating well, and the water's crystal clear. The water chemistry is pretty much exactly where it was when I checked it in September.

I'm going to keep it under observation for another week before thinking about reintroducing new fish. And when I do, I definitely won't get another pleco or clown loach despite how beautiful they are.

Corydoras are very susceptible to bacterial infections, particularly pop-eye and fin rot. I would hazard a guess that one of your Cory's readily developed a bacterial infection from eating remnants of old food on the substrate and when it became deceased the other Cory's would have nibbled at its corpse which would have caused the bacterial infection to be ingested and therefore propagated. This would explain the high degree of deaths among your Corydoras.

My advise would be to keep your substrate very clean and have a net ready to remove any food which has made its way to the bottom that is too big for the Cory's to eat OR which they won't eat and is likely to rot.

I would recommend a 90% water change to cleanse the water of any excess ammonium or bacteria.