Bronze Corydora female is panting

[HoneyFish]

I noticed a couple of days ago that my big female bronze cory was panting. Normally they move their mouth/gills when eating food, but she does this constantly now and has never shown this behavior before. Other than her mouth/gills moving rapidly, she also goes up for air a bit more than usual- but she doesn't constantly swim at the top or anything.
The only different thing I have done recently is switch my filter cartridge out, but I still have my old filter cartridge in the tank for now, until the newer one grows good bacteria. I have done this same method before and never had issues with her or any other fish. She still swims around and is active and eating.

Info of the tank:
10 Gallon tank.
I have had this tank set up since around June of 2021.
I have a heater that keeps the tank at 79.2. (I would like the temp to be a little lower but can't seem to get a heater to keep the temp at what I want, which is 75.)
and a Top Fin corner filter.
My API 5 in 1 test strip results are always as follows: The GH is below 30, the KH is about 40, the PH is 7.0, the Nitrite is 0 and the Nitrate is definitely under 20, maybe even 0. These are the test results now, and what they have always been.

I have a bubble wall that is on constantly, and is actually an upgrade from the small air-stone I had before. I did an almost 30% water change last night and yet she hasn't stopped the rapid movement of her gills.
I have no idea what may be causing this stress all of the sudden. I got her when she was very small and she is absolutely huge- and has been big for a while now. She has never moved her gills this fast.
I also have other corydoras in the tank and none of them have rapid gill movement, except for her.
If someone could please help me figure out what's wrong, I would so appreciate it. I love her so much and want her to be okay.

I'm not sure if I can post videos of her on here, but I will if anyone needs it for reference.

 

[PheonixKingZ]

What are your ammonia levels? Usually gasping indicates high ammonia levels or low oxygen levels. I would do a 30% water change and see if her breathing improves.

Is it just her in the tank?

 

[itiwhetu]

The higher the temperature the lower the oxygen level. 79 degrees is generally too high for Cory's to be happy.

 

[HoneyFish]

How low can I

The higher the temperature the lower the oxygen level. 79 degrees is generally too high for Cory's to be happy

How low do corydoras like the tempt to be? I've seen some conflicting answers when I google what temperature they thrive in. I also can't find a heater that keeps the temp at 75, which is what I want it to be. The heaters I have tried always say they will keep the water at 75 and they always warm the tank up more than that.

 

[HoneyFish]

What are your ammonia levels? Usually gasping indicates high ammonia levels or low oxygen levels. I would do a 30% water change and see if her breathing improves.

Is it just her in the tank?

I'm not sure of my ammonia levels, because I only have a test kit that tells me the GH, KH, PH, Nitrate and Nitrite. But I did just do a 30% water change and her gill movement hasn't slowed down at all.
I also have one other bronze cory and a panda cory in with her- which makes a total of 3 fish in the tank all together. And I know I should probably have at least one more cory, but they have done so well and seemed to be thriving with just the 3 of them all this time.

 

[foxgirl158]

I'm not sure of my ammonia levels, because I only have a test kit that tells me the GH, KH, PH, Nitrate and Nitrite. But I did just do a 30% water change and her gill movement hasn't slowed down at all.
I also have one other bronze cory and a panda cory in with her- which makes a total of 3 fish in the tank all together. And I know I should probably have at least one more cory, but they have done so well and seemed to be thriving with just the 3 of them all this time.

You need an ammonia test kit ASAP. Ammonia is highly toxic to fish and, if built up enough, can kill them in less than a day if not taken care of. Also, the test strips are highly inaccurate. I and many others ask you to please invest in an API Master test kit, linked below. Your fish will thank you.
*edited to fix a typo
https://www.amazon.com/dp/B000255NCI/?tag=ff0d01-20

 

[itiwhetu]

How low can I

How low do corydoras like the tempt to be? I've seen some conflicting answers when I google what temperature they thrive in. I also can't find a heater that keeps the temp at 75, which is what I want it to be. The heaters I have tried always say they will keep the water at 75 and they always warm the tank up more than that.

What is your room temperature in theory your tank should be able to be kept at your room temperature, most good quality heater/thermostats start at around 22 degrees C. Never rely on the numbers on the dial, in forty years I have never had one that is accurate, you need an independent thermometer and then set your heater/thermostat to that ignoring what the numbers say. For Cory's 24-25 degrees C is perfect

 

[HoneyFish]

What is your room temperature in theory your tank should be able to be kept at your room temperature, most good quality heater/thermostats start at around 22 degrees C. Never rely on the numbers on the dial, in forty years I have never had one that is accurate, you need an independent thermometer and then set your heater/thermostat to that ignoring what the numbers say. For Cory's 24-25 degrees C is perfect

I have a heater and a completely separate thermometer already. My thermostat stays at 71 F. in my house, so that may be too cold for them? I can't seem to find an affordable heater that stays at 75 F.

 

[HoneyFish]

You need an ammonia test kit ASAP. Ammonia is highly toxic to fish and, if built up enough, can kill them in less than a day if not taken care of. Also, the test strips are highly inaccurate. I and many others ask you to please invest in an API Master test kit, linked below. Your fish will thank you.
*edited to fix a typo
https://www.amazon.com/dp/B000255NCI/?tag=ff0d01-20

Thanks so much! I just ordered an ammonia tester, though I don't know when it will arrive. All the stores in my area were out of stock so I ordered one from Chewy. Fingers crossed it gets here fast.

 

[itiwhetu]

I only ever use Eheim Jager heaters in my tanks adjustable from 18-34 degrees C

 

[HoneyFish]

I only ever use Eheim Jager heaters in my tanks adjustable from 18-34 degrees C

I just bought one of those and I can pick it up tomorrow. Hopefully I'll like it. Thanks for the suggestions

 

[AdoraBelle Dearheart]

How often do you usually do water changes, and roughly how much of the water do you usually change when you do?

Any other fish in there?

Have you added anything to the tank recently?

 

[HoneyFish]

How often do you usually do water changes, and roughly how much of the water do you usually change when you do?

Any other fish in there?

Have you added anything to the tank recently?

I change about 25% to 30% of their water every Sunday. She's kept in a 10 gallon with two other corydoras.
No new fish, and nothing new has been added to the tank, other than a new filter cartridge. I did leave the old filter cartridge in the tank as well, until the new one grows some good bacteria.

 

[AdoraBelle Dearheart]

I change about 25% to 30% of their water every Sunday. She's kept in a 10 gallon with two other corydoras.
No new fish, and nothing new has been added to the tank, other than a new filter cartridge. I did leave the old filter cartridge in the tank as well, until the new one grows some good bacteria.

So there are no other fish in there apart from the three cories?

 

[HoneyFish]

So there are no other fish in there apart from the three cories?

That's correct