Corydoras dying, idk what to do

[Bicyclemaster]

Hey there. I have a 10G that's been running for about a year and a half. It's also heavily planted. At this poin I don't know what to do because when I started I got 7 panda cories, 5 of them died along the way and I never understood why.
My water is 24⁰C (75 F).
Nitrite and ammonia are 0
nitrate si 5-10
pH is 7.5
KH is 10 and GH is 8
It's always been like this, only one time when I didn't do a water change for 2 weeks the nitrate reached 20. I normally do 10% changes.
Some time ago I got another 7 cories, now from a breeder, because I thought that the first ones which I had bought from a petshop might had been sick (most people here say that the fish they get from petshops die). These new cories were fine for 1 month than they started dying one by one about one per week. 5 have died now and I really don't know what to do. Oxygen cannot be a problem beacuse I have both an air pump and an HOB running.
i thought about some possibilities: because there are so many plants maybe there isn't enough air circulation at the bottom of the tank and the water is more toxic. My substrate is aquasoil with a thin layer of sand on top and sometimes there are bubbles of air in the sand which from what I know can be very toxic to fish.
One last thing, there are 2 nerite snails in that tank that have been there since the beginning and a female betta. I'm sure the betta isn't the problem because I've never seen her attack any of the fish or snails.

 

[Bicyclemaster]

One other thing about the new cories is that since the moment I got them they never did their actual job. They were mostly going up and down the tank, on plants, being overall very active but not scavenging the bottom of the tank. The pandas that I got in the beginning were normal scavengers.

 

[Naughts]

Their job? Corydoras are not scavengers, just substrate feeders. What are you feeding them?

I would suggest a 15g+ is more suitable. They also need to be in a group although I can see you would want to fix the problem before adding more.

Please add photos.

 

[Bicyclemaster]

Their job? Corydoras are not scavengers, just substrate feeders. What are you feeding them?

I would suggest a 15g+ is more suitable. They also need to be in a group although I can see you would want to fix the problem before adding more.

Please add photos.

I know they don't have a specifiv job or anything . I just mean that they were not really staying on the substrate, more just going around in the tank. I feed them sinking pellets and frozen bloodworms from time to time. The photos are about 2 months old because I'm not home right now but the only differece is that there are a bit more plants.

 

[Naughts]

OK, can you be more specific about the type of sinking pellet? I know sometimes people don't know that cories need protein hence just feed algae wafers or similar.
I can't spot the cories in the photos so maybe post pictures when you are home?
The tank looks to have a small footprint for cories.

 

[Byron]

There are a couple things to note here. I am not suggesting any one of these is killing the cories, but it is most certainly possible that the issues are negative in themselves and the cumulative effect is more impacting.

First, the substrate. Plant "soils" should never be used in a tank with cories. These specialized plant substrates can have a serious issue with bacteria. And the cories being on the substrate and (normally) digging into it, is why they are much more affected that would upper fish be. The bubbles may be a clue to this, not in the sense of anaerobic but just bacteria that live in the substrate. Cories will want to dig down very deep, so even having sand on top is not sufficient. The underlying bacteria bed is the problem.

The second thing is the food. Cories are very specific eaters. Foods high in protein cause serious issues. Knowing exactly what you are feeding them will help. But I'll cut to the chase and suggest (1) Fluval Bug Bites, and (2) Omega One shrimp pellets. These two foods replicate the exact diet of this fish in their habitat--insects and insect larvae and crustaceans. The protein levels are lower than most upper fish foods, without the fats. Not all "shrimp pellets/tabs" are as good, the brands here is extremely important.

CO2 during night when it is naturally rebuilding in the substrate can be a problem, I have seen this myself in planted tanks, but provided the HOB filter is creating good surface disturbance day and night, this should not itself be an issue.

I see no issues with the water parameters.

 

[Bicyclemaster]

There are a couple things to note here. I am not suggesting any one of these is killing the cories, but it is most certainly possible that the issues are negative in themselves and the cumulative effect is more impacting.

First, the substrate. Plant "soils" should never be used in a tank with cories. These specialized plant substrates can have a serious issue with bacteria. And the cories being on the substrate and (normally) digging into it, is why they are much more affected that would upper fish be. The bubbles may be a clue to this, not in the sense of anaerobic but just bacteria that live in the substrate. Cories will want to dig down very deep, so even having sand on top is not sufficient. The underlying bacteria bed is the problem.

The second thing is the food. Cories are very specific eaters. Foods high in protein cause serious issues. Knowing exactly what you are feeding them will help. But I'll cut to the chase and suggest (1) Fluval Bug Bites, and (2) Omega One shrimp pellets. These two foods replicate the exact diet of this fish in their habitat--insects and insect larvae and crustaceans. The protein levels are lower than most upper fish foods, without the fats. Not all "shrimp pellets/tabs" are as good, the brands here is extremely important.

CO2 during night when it is naturally rebuilding in the substrate can be a problem, I have seen this myself in planted tanks, but provided the HOB filter is creating good surface disturbance day and night, this should not itself be an issue.

I see no issues with the water parameters.

I feed them tetra wafers and tabimin. Sadly, these are the only higher quality bottom dweller food I can find where I live. Is there a certain procentage of protein that is better for cories? The ones I have are about 43-45%.

As for the co2, i specifically bought an air pump to be sure that there is enough oxygen because the HOB didn't create enough surface agitation.

I don't think I'll get anymore because I really don't want to torture any more fish. I really do like bottom dwellers and how they interact in their environment tho. Maybe I'll try adding some tetras or something but I have to be mindful of the betta.

 

[Byron]

In the absence of anything else, I still suggest the substrate is the most likely issue.

As for the food, those are high in protein which is certainly not good, but here again this would not be the cause of such rapid deaths. The brands I mentioned previously have in the low 30 percentage of protein, which makes a big difference. The Bug Bites bottom feeder formula for example is 32% protein.

 

[Bicyclemaster]

I don't think I can do anything about the substrate because it would be hard to grow plants with inert substrate. I will just stop getting bottom dwellers and hope that the ones I still got will be okay. I will also try to find a food with 30% protein to feed the ones I have left.
Thanks a lot for the info.

 

[Byron]

I don't think I can do anything about the substrate because it would be hard to grow plants with inert substrate. I will just stop getting bottom dwellers and hope that the ones I still got will be okay. I will also try to find a food with 30% protein to feed the ones I have left.
Thanks a lot for the info.

The plants are certainly lovely in the photos, but I don't know how much of this could actually be attributed to the aquasoil. And there is the very real issue of what these substrates do to the fish. Substrate level fish are down there as I said earlier and thus more prone to the problems, but that does not mean there may not be issues for upper fish. I and some other members here are of the view that one should decide whether fish or plants are the focus. If fish, then one must be very cautious with plant additives of any sort, as they get inside fish. That does not mean one cannot have plants in a tank with fish, but the plants must be secondary to the health of the fish.

All aquarium plants will grow very well with an inert substrate of soft sand. When needed, fertilization can be added in the form of substrate tabs for rooted plants that benefit from this (not all rooted plants really do benefit), and/or liquid for plants that will gain no benefit whatsoever from substrate fertilization, such as floating plants (your water lettuce).

Again let me be clear, I am not saying your substrate is definitely killing the fish. But I am saying that is is a very real possibility that has to be recognized.

 

[Crystal90]

I'll throw in my 2¢. I don't think 10% water changes are enough. Fresh water is always a good thing.

 

[Byron]

I agree, and sorry I missed this previously as it is very important. With cories in the tank, it is often a good practice to do a good clean of the substrate at each water change. However, you are somewhat hampered here by the plant substrate. I don't know what sort of mess might occur if you poke into it as one can with inert substrates, so be careful. By "mess" I would be more concerned with bacteria.