Pygmy corys mysteriously dying/disappearing

I've had 10 pygmy cory catfish with 10 harlequin rasboras for almost 2 years now in a 20 gallon tank. Over that time I've lost 2 catfish and now a third one is missing and presumed dead. The other two also went missing and when I tore the tank apart to move it I confirmed their death when I could not locate them. The tank sits around 76 degrees depending on the house temp, it uses an aquaclear 20 or 30 (I can't remember I purchased it with the tank).
I do 50% water changes weekly although I have been lapsing on that recently and have multiple plants in the tank which I dose with thrive When changing the water I always tempt match it and the nitrates rarely if ever have surpassed 80 before a water change. My only thoughts for this recent death would be due to my lapse in water changes because of some outside life stuff that prevented me from doing consistent water changes, most water changes were 50 percent but they were inconsistent and not enough. Another possible cause could be an unvaried diet, they are often fed Aqueon shrimp pellets and only ocassionaly frozen bloodworms. Is this fatality rate normal for pygmy corys? Did I just get some weaker fish or is it a sign of a greater issue within the tank?

 

Have you checked your filter? I know it's a long shot but sometimes fish get stuck in HOB filters.
Either that or they're being eaten as soon as they pass away.
Do you know your water parameters?

 

Have you checked your filter? I know it's a long shot but sometimes fish get stuck in HOB filters.
Either that or they're being eaten as soon as they pass away.
Do you know your water parameters?

I do not know exactly since I've only been using strips lately as they are convenient and the tank is pretty self sufficient due to the amount of plants. I cleaned the filter a week ago and know the fish isn't in there. The GH appears to be about 30 with the KH somewhere between 40 and 80. The ph is between 7.5-8 and the nitrates are between 40-80.

 

Do a couple of back to back water changes to get the nitrates down and see if that helps. Corydoras can be pretty sensitive

 

Do a couple of back to back water changes to get the nitrates down and see if that helps. Corydoras can be pretty sensitive

Would they be considered a more suitable fish species for a bigger tank or one for more consistent water changes? and is this death rate normal or is it a fish keeper error?

 

A lot of people like pygmy corydoras because they are good for smaller tanks. (And they're cute)

Consistent water changes are important for every size tank but larger tanks tend to be more stable so if you miss one it's not a big deal.

Regular deaths are not normal unless there's there's something wrong like the fish are sick, being bullied or old. Speaking of, have you noticed any of your corydoras acting weird before they dissapeared or have you noticed the rasboras chasing them?

If not I think (and don't quote me on this) pygmy corydoras have a pretty short lifespan compared to the larger species. They may just be dying of old age.

 

A lot of people like pygmy corydoras because they are good for smaller tanks. (And they're cute)

Consistent water changes are important for every size tank but larger tanks tend to be more stable so if you miss one it's not a big deal.

Regular deaths are not normal unless there's there's something wrong like the fish are sick, being bullied or old. Speaking of, have you noticed any of your corydoras acting weird before they dissapeared or have you noticed the rasboras chasing them?

If not I think (and don't quote me on this) pygmy corydoras have a pretty short lifespan compared to the larger species. They may just be dying of old age.

I'm thinking it's due to old age as they generally have a lifespan of 3 years. I haven't seen any odd behavior except that one of the ones who died never really stayed with the rest of the corydoras but that was the entire duration I had him which was a 6 months at the least. I have no idea how the rest behaved because I never looked that closely. The tank at the very start of it's lifespan picked up some internal parasitic worms from two honey gourami that passed away but I treated it and never saw symptoms in any of the other fish, not to mention the mortality rate would be way higher then 3 fish in two years. The rasboras are another concern of mine due to how they are so shy and sit in a corner all day, but that makes me certain they never messed with the corydoras, as the corydoras would sometimes go up to school with the rasboras. The rasboras have brigh-ish colors which makes me think they aren't stressed, maybe they need more?

 

A lot of people like pygmy corydoras because they are good for smaller tanks. (And they're cute)

Consistent water changes are important for every size tank but larger tanks tend to be more stable so if you miss one it's not a big deal.

Regular deaths are not normal unless there's there's something wrong like the fish are sick, being bullied or old. Speaking of, have you noticed any of your corydoras acting weird before they dissapeared or have you noticed the rasboras chasing them?

If not I think (and don't quote me on this) pygmy corydoras have a pretty short lifespan compared to the larger species. They may just be dying of old age.

secondary thing I thought of is that my water runs through copper pipes, could that cause it?

 

secondary thing I thought of is that my water runs through copper pipes, could that cause it?

I'm not sure, it might affect invertebrates more than fish

 

I'm not sure, it might affect invertebrates more than fish

Most peoples plumbing is copper, not to mention python hoses use brass fittings so I am assuming the amount is negligible but who knows.

 

Properly up to date copper pipes shouldn’t leech copper. You could test for copper in your water?

 

Maybe 2 years of high nitrates have taken their toll, or is that more of a recent ocurance?

 

Maybe 2 years of high nitrates have taken their toll, or is that more of a recent ocurance?

It's more of a recent occurrence, I used to keep the nitrates too low for my plants, around 20 nitrates. I had at least one of the cories if not two of them disappear during that time.

 

20ppm is more than the plants need. While high nitrate may not hurt your plants, it's not good for fish in the long term IMO. My tanks never get that high. I have 2 planted tanks. One stays at 5ppm and the other at zero. They both get fertilizer 3x/week. I don't think nitrate is your limiting nutrient.

Which Thrive are you using?

Also, overdosing ferts will not help much if you don't also increase CO2 and light and balance the three.

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The science does not back my concern for high nitrates. One study of chronic nitrate exposure showed that at nitrate levels of 400ppm rainbow trout begin to show increased mortality.

R. Trout fry and juveniles are much more sensitive to nitrate with levels under 100ppm causing death, deformity and other problems.

"chronic exposure" in these studies usually means a few weeks or 2-3 months. That's partly because these tests are often for the aquaculture industry which does not care how it effects the fish over many years because the fish will be slaughtered as soon as they are mature. I want my fish to live a decade or more. We don't see studies looking at that side of it.

Nitrate in aquaculture systems can be as high as 2000ppm and surface waters have been measured at near 400ppm in places.

Natural waters (no human influence) have a low nitrate levels, generally below 10ppm, so I guess I like to use that as my guide. Low levels of toxin exposure over a lifetime often affect health for many organisms, so I don't see that fish would be any different. That's just my working therory...

 

20ppm is more than the plants need. While high nitrate may not hurt your plants, it's not good for fish in the long term IMO. My tanks never get that high. I have 2 planted tanks. One stays at 5ppm and the other at zero. They both get fertilizer 3x/week. I don't think nitrate is your limiting nutrient.

Which Thrive are you using?

Also, overdosing ferts will not help much if you don't also increase CO2 and light and balance the three.

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The science does not back my concern for high nitrates. One study of chronic nitrate exposure showed that at nitrate levels of 400ppm rainbow trout begin to show increased mortality.

R. Trout fry and juveniles are much more sensitive to nitrate with levels under 100ppm causing death, deformity and other problems.

"chronic exposure" in these studies usually means a few weeks or 2-3 months. That's partly because these tests are often for the aquaculture industry which does not care how it effects the fish over many years because the fish will be slaughtered as soon as they are mature. I want my fish to live a decade or more. We don't see studies looking at that side of it.

Nitrate in aquaculture systems can be as high as 2000ppm and surface waters have been measured at near 400ppm in places.

Natural waters (no human influence) have a low nitrate levels, generally below 10ppm, so I guess I like to use that as my guide. Low levels of toxin exposure over a lifetime often affect health for many organisms, so I don't see that fish would be any different. That's just my working therory...

I use thrive c, I had a hornwort plant in my twenty gallon with an aqua clear fifty and it would shed constantly and stay shedded more then it would have needles. My only guess for what caused this was either too high of flow or too low of nutrients or possibly it not liking my water parameters, but I've heard they basically wont mind any type of water.

Properly up to date copper pipes shouldn’t leech copper. You could test for copper in your water?

How would I test for copper?

 

How would I test for copper?

There’s an API test kit for copper, but you could probably ask on local fishkeeping page. I know there’s one or two test kits that have been passed around our neighborhood for about two years now.