Pygmy Corydoras dying for unknown reason

Hi folks,

I got a school of 11 pygmy corydoras last week and introduced them to my 100L tank after acclimatization, but they have been dying one at a time everyday/other day.
I have been recommended Hikari Sinking Carnivore Pellet and microworms. When I drop the microworms (scooped these up from the wall of my culture container and mixed them with tank water before using a dropper to drop them into the tank), some of them swim around the area I think the worms drop, but I cant tell if they are feeding or not. Meanwhile, they totally ignored the sinking pellet. They forage around it, but not on it.

I am on the verge of not knowing what to do beside trying different food... Got Repashy Community Plus on the way.
Since I am down to about 6 of them, I brought in another 10 to keep their numbers high to try to reduce stress as I do have some voracious eater guppies who just eat anything. The guppies in this tank are not as voracious and rowdy as my other miscellaneous guppies in a different tank, who fights over food and stir up all the particulates sitting on the substrate.

Not sure if I am doing something wrong so I am hoping to get some guidance. Appreciate any and all help/advice.
I am not exactly new to the hobby and I never had a problem with keeping things alive after passing my initial learning curve.


Tank
What is the water volume of the tank? ~100L
How long has the tank been running? 1.5 months using substrate transferred from a mature/cycled tank (6 months+ old)
Does it have a filter? Big sponge filter at the back
Does it have a heater? Have a Fluval e200 heater
What is the water temperature? consistently 21 degrees Celcius
What is the entire stocking of this tank? (Please list all fish and inverts.)

1. Rummynose Tetras (3)
2. Blue Grass Guppies (14 females, 4 males)
3. Otocinclus Catfish (7)
4. Fire Red Cherry Shrimp (~12)
5. Pygmy Corydoras (6 + 10)

Maintenance
How often do you change the water? Once a week
How much of the water do you change? ~15%
What do you use to treat your water? Seachem Prime on water change. Seachem Potassium (5-8 mL about a week ago, never use it before). Added some crushed eggshells and wondershell to help stabilize the pH level.
Do you vacuum the substrate or just the water? I vacuum the substrate around the front open area only. I did have a secondary water pump active at some point to generate some current and promote flow.

*Parameters - Very Important
Did you cycle your tank before adding fish? Yes
What do you use to test the water? AquaOne 5-in-1 test strip that checks for Nitrate, Nitrite, pH, GH, KH. Also have Seachem Ammonia and pH censor active.
What are your parameters?
Ammonia: <0.02 ppm
Nitrite: 0 or close to 0 ppm
Nitrate: 0 or close to 0 ppm
pH: 6.6
KH: 40 ppm
GH: ~170 ppm
TDS (using those portable TDS meter): 320 ppm

Feeding
How often do you feed your fish? Twice a day. A mix of New Life Spectrum Optimum Flakes and boiled peas once a week for the guppies. Recently started adding microworms and/or crushed Hikari sinking carnivore pellet (2x), small pinch of spinach powder every now and then. I haven't start using blanched veggie such as spinach and zucchini in this tank yet.
How much do you feed your fish? A pinch of flakes (I usually crush them to smaller size so that the guppies wont overeat as easily)
What brand of food do you feed your fish? New Life Spectrum Optimum Flake, Hikari Sinking Carnivore Pellet.
Do you feed frozen? Not yet in this tank.
Do you feed freeze-dried foods? Not yet in this tank.

Illness & Symptoms
How long have you had this fish? My Pygmy Corydoras arrived about Thursday last week.
How long ago did you first notice these symptoms? Around Sunday.
In a few words, can you explain the symptoms? No obvious symptoms other than finding dead Pygmy Corydoras in the morning.
Have you started any treatment for the illness? Not sure if it is an illness or not, so other than exposing them to Melafix while acclimatizing, I have not used any medication
Was your fish physically ill or injured upon purchase? No.
How has its behavior and appearance changed, if at all? No. They school occasionally (about 10-20% of the time) in pack of about 5-7, then the remaining of the time seems to be split evenly between foraging through the substrate and just sitting on the substrate.

 

consistently 21 degrees Celcius

Lower end of the temperature range for these fish.

Added some crushed eggshells and wondershell to help stabilize the pH level.

You don't have to mess with that for Corydoras. What are the tap-readings without that?

Blue Grass Guppies (14 females, 4 males)

How often do you feed your fish? Twice a day. A mix of New Life Spectrum Optimum Flakes and boiled peas once a week for the guppies. Recently started adding microworms and/or crushed Hikari sinking carnivore pellet (2x), small pinch of spinach powder every now and then. I haven't start using blanched veggie such as spinach and zucchini in this tank yet.
How much do you feed your fish? A pinch of flakes (I usually crush them to smaller size so that the guppies wont overeat as easily)

Very likely the Corydoras get too little food. With that competition you will have to target feed or feed more.
Veggies are largely irrelevant for Corydoras btw.

How long have you had this fish? My Pygmy Corydoras arrived about Thursday last week.

So you haven't quarantined them and they are relatively new to the tank. So it's absolutely possible they have had something before purchase that only now shows up.

I see tseveral problem factors possibly contributing. Temp, competition, food amounts and the fact they are recently introduced. Except the feeding factor most are neglectable by themselves, but can become a problem in combination.

 

Lower end of the temperature range for these fish.

You don't have to mess with that for Corydoras. What are the tap-readings without that?


Very likely the Corydoras get too little food. With that competition you will have to target feed or feed more.
Veggies are largely irrelevant for Corydoras btw.

So you haven't quarantined them and they are relatively new to the tank. So it's absolutely possible they have had something before purchase that only now shows up.

I see tseveral problem factors possibly contributing. Temp, competition, food amounts and the fact they are recently introduced. Except the feeding factor most are neglectable by themselves, but can become a problem in combination.

Temperature wise, I may have to add an extra heater if it is too low. The heater's default setting is 24 degrees Celcius, but it is pretty cold here and it is struggling to get it to stay at 22 degrees Celcius even though I thought this heater's manual says that this heater is sufficient for up to 200L.

Regarding pH level, I added a big piece of driftwood about 2 weeks ago, which resulted in some changes to pH level to about 6.4ish (can't quite remember now), but it is now stable at 6.6and hasn't change since.

I do employ targetted feeding, so I distract the guppies with flakes in front, then using a dropper to put in crushed sinking pellet, crushed flakes, microworm, and sometimes a bit of hikari first bite on the side. Looks like I might have to do more and maybe even late-night feeding in the dark.

Regarding quarantining, it is an oversight on my end this time as I usually quarantine them. They do not appear to exhibit any worrying behavior or signs, so I took a leap of faith this time.

Thanks for the insight. Definitely never skip the quarantine step again in the future.

 

Temperature wise, I may have to add an extra heater if it is too low. The heater's default setting is 24 degrees Celcius, but it is pretty cold here and it is struggling to get it to stay at 22 degrees Celcius even though I thought this heater's manual says that this heater is sufficient for up to 200L.

As I don't know the model I can't say much. 24°C would be a good temp.

Regarding pH level, I added a big piece of driftwood about 2 weeks ago, which resulted in some changes to pH level to about 6.4ish (can't quite remember now), but it is now stable at 6.6and hasn't change since.

What was the level before the 6.4? Also, a difference of 0.2 is nothing. The only fish I'd be concerned about are the livebearers, though very adaptable for hardwater fish, there is a low end to their range that is significantly higher than the low end of the parameter range of the amazonian fish. Rummynose, Otos and Corydoras could be maintained in 100% RO, which is closer to their natural conditions. With the humic substances ("tannins") you have plenty of low-range (4-6) buffer capacity.

I do employ targetted feeding, so I distract the guppies with flakes in front, then using a dropper to put in crushed sinking pellet, crushed flakes, microworm, and sometimes a bit of hikari first bite on the side. Looks like I might have to do more and maybe even late-night feeding in the dark.

While the foods you feed will settle on surfaces which is great for your otocinclus, a lot will likely not reach the bottom and the places Corydoras prefer to forage at. More directly sinking foods are probably a good thing.

Regarding quarantining, it is an oversight on my end this time as I usually quarantine them. They do not appear to exhibit any worrying behavior or signs, so I took a leap of faith this time.

Well... what can I say.

Keep an eye on the situation and let's hope you didn't introduce something nasty.

 

As I don't know the model I can't say much. 24°C would be a good temp.


What was the level before the 6.4? Also, a difference of 0.2 is nothing. The only fish I'd be concerned about are the livebearers, though very adaptable for hardwater fish, there is a low end to their range that is significantly higher than the low end of the parameter range of the amazonian fish. Rummynose, Otos and Corydoras could be maintained in 100% RO, which is closer to their natural conditions. With the humic substances ("tannins") you have plenty of low-range (4-6) buffer capacity.


While the foods you feed will settle on surfaces which is great for your otocinclus, a lot will likely not reach the bottom and the places Corydoras prefer to forage at. More directly sinking foods are probably a good thing.


Well... what can I say.

Keep an eye on the situation and let's hope you didn't introduce something nasty.

Noted on the temperature.

My water here is usually at around 6.6 pH range.

When I target feed, I use a dropper and watch where the food lands. These crushed particulate are usually very good at reaching the bottom, but I will swap to the use of turkey baster to get closer to the substrate.

Another option I am exploring now is adding more bushy plants as my guppy have a habit of coming down to forage around the substrate once they finish feeding at the top.

 

I also had problems keeping Corydoras pygmaeus alive, in my case in a soft water tank with pencil fish and an apisto, with each species quarantined separately for 8 weeks. I started with 9, and they eventually winnowed down to 3 or 4 in less than a year.

If you do a search on Planet Catfish, you'll find that it's a somewhat common occurrence with this species, without an exact resolution. Not a satisfying answer, but at least you're not alone. One of my LFS doesn't carry them for this reason.

My ideas are similar to what's been suggested above.

1. Could be outcompeted for food. In my case, I had amano shrimps that pretty aggressively went after the shrimp pellets. I switched to a brand of pellet that breaks apart into small pieces easily. The Wardley brand does this, while the Omega One is more of a dense, hard pellet. You could try crushing it into smaller pieces. I also started feeding microworms, which they really liked. I didn't think the shrimp would go after those, but they did also. If I kept pygmy corys today, I'd probably offer grindal worms and Wardley shrimp pellets.

2. Could be that they were carrying some disease already, and it expressed more fully under stress.

 

My ideas are similar to what's been suggested above.

1. Could be outcompeted for food. In my case, I had amano shrimps that pretty aggressively went after the shrimp pellets. I switched to a brand of pellet that breaks apart into small pieces easily. The Wardley brand does this, while the Omega One is more of a dense, hard pellet. You could try crushing it into smaller pieces. I also started feeding microworms, which they really liked. I didn't think the shrimp would go after those, but they did also. If I kept pygmy corys today, I'd probably offer grindal worms and Wardley shrimp pellets.

2. Could be that they were carrying some disease already, and it expressed more fully under stress.

Noted.
I thought I had done my due diligence in researching the species and compatible tank mates. Although in hindsight, I should have taken into consideration how much of a voracious eater my guppy females are. They eat the ones floating, then fight over the ones at the bottom. You would almost think that they are some sort of betta/tiger barb/gourami in guppy skin if you watch them go after a sinking pellet.

I now have 2 heaters in the tank and the water temperature is going up to 22.5 degrees Celcius as of this morning (it is around 7 degrees Celcius in a heritage-listed cottage, so the indoor temperature is quite, probably as cold as it is outside at night). Will continue to watch how this goes and get a new heater if I can't get it to 24 degrees Celcius.

Regarding food, I am getting a turkey baster to pump crushed sinking pellets at night when everyone else is sleeping. I dropped like 6 of them whole last night and it is barely touched by the morning. Never seen them much on these during the day either. Will continue to monitor their situation closely and ensure that I am giving them my very best care to make up for my oversight at the start. Thanks!


Edit:
As an addition, Hikari Sinking Carnivore Pellets are quite breakable. I can literally crush it with two fingers, which is what I did, before mixing it with some aquarium water and pump/pour it down to the substrate. I will mix in microworm, spinach powder (occasionally), Hikari First Bites, and Optimal Flakes (Krill/garlic mix)

 

Just wanted to bring this to a close. Water temperature up to 25 degrees Celcius. My morning routine now is I put in crushed sinking pellets mixed with Optimum Flakes on the side of the aquarium where the Pygmy Corydoras like to hang out, then distracting the Guppies with flake food on the surface. My night routine is another cup of aquarium water with the same mix and pouring it over the same spot while the guppies are sleeping.

No more dead Pygmy Corydoras since Wednesday. Fingers crossed it stays this way as I am confident they are better fed now.

Thanks for all the advice and input above