Fish behaving oddly, potentially dying of stress / nitrite exposure?


105L tank
Run for almost 3 months
Sponge filter
Aquarium heater
25 degrees celcius
3 peppered cory cats, 7 rummynose tetras, 6 glowlight tetras, 1 gold nugget pleco

Water changed weekly
30-40% volume
Tetra AquaSafe water conditioner
Don't clean substrate

*Parameters - Very Important
Cycled tank completely for four weeks prior to adding fish
Using API test strips to test water parameters
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.
Ammonia: (doesn't tell me)
Nitrite: 0ppm
Nitrate: 20ppm
pH: 7.0

Feed fish daily
One small pinch of catfish pellets, one small pinch of tropical fish flakes, one algae wafer daily (6 times a week)
King British (exclusively)
Do not feed frozen
Do not feed freeze dried foods

Illness & Symptoms
Had the fish for just under 2 months
Noticed symptoms three days ago
Corys are darting to the surface gasping for air, glowlights are lethargic, handful of rummynoses struggling to right themselves in water + floating vertically nose down
Fed deshelled, defrosted frozen peas in case of swim bladder disease and moved filter to agitate surface more to try to bring in more oxygen
All fish were healthy when purchased
The corys were never this frantic or irritable, some rummynoses are pale and lacking red colouration, rummynoses are suddenly floating vertically

Explain your emergency situation in detail.

I'm VERY new to fishkeeping, my tank has been set up for a little under 3 months; 4 weeks fishless cycle with 2 live anubias nanas and one small mesh rectangle with java moss on it. After my fishless cycle I added three peppered cory cats and everything was fine until I added seven rummynose tetra and six glowlight tetra at once because the local fish store recommended that this was safe. I very quickly realised my mistake with adding so many fish at once as the nitrites immediately soared above 10ppm on my API test strips. These numbers did not go down for almost 3 weeks, I managed to have no fish die, but I am INCREDIBLY concerned for their long term health as they have recently began to behave in increasingly erratic ways. At one point during this one of my anubias plants died and started decaying all within a two hour period from when I had last saw it, completely healthy looking. I removed it as quickly as possible and have had no other plant issues. I don't plan on adding any additional fish to the tank because I'm worried I may be overstocking it.

Now that the nitrites are back down to 0, I added one more fish last week, a gold nugget pleco. I'm incredibly worried about this as I went to a different store this time, and they assured me that plecos and corys get along fine, but I am now being told otherwise. I worry for my catfishes' health. They have began to scrape gravel along the glass and as of yesterday they have started to frantically swim to the surface, gulp at the air, and then hide. One of the cory catfish is rather large and I have noticed eggs on the glass and I'm not sure if this is related. The pleco appears to be doing fine but I worry that it may outgrow the tank; the store told me it would grow to no larger than 6 inches but I am now being told otherwise. I don't know if the corys are stressed because of the pleco or exposure to nitrites.

My glowlight tetras rarely eat or move and haven't done so since I got them. they tend to stay together underneath the filter in the back corner. My rummynose tetras are more active but as of three days ago a couple have began to float vertically with their noses pointed down and tails up, and seem to have trouble righting themselves. One of the rummynose tetras has had no red colouration on it's head for the past week. I fed them some crushed, deshelled and defrosted peas about 10 minutes before writing this as recommended online but I'm unsure as to whether this is swim bladder disease or some other issue. Today one of my smaller catfish also chased some of my tetras around briefly and appeared to be trying to nip at their fins which I've been told is stressful for all fish involved.

Are my fish just heavily stressed, or is this all caused by prolonged nitrite exposure? I tried as much as I could to lower the levels as fast as possible and I am honestly very surprised that I didn't lose any fish during this time. I'm struggling to find information online to process all of this and I thought I would make an account on some forums with people that know what they're doing and see if I could get any insight into what could potentially be happening. I tested my water with the API test strips right before writing this so the parameters are up to date. I apologise for the amount of rookie mistakes I've made so far, this is my first tank, any help with trying to fix this is greatly appreciated!


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Sounds like you have started off much better than many of us did :)
A couple of things come to mind-- if your cories are spawning, they are in good shape! The tetras would all like some more plants-- if you can see all the fish at once, there aren't enough. That will help them feel safe so they can show their colours and come out more. Right now they are trying to blend into the background. As for the rummies acting sick...they are not an easy fish. Maybe someone can help save yours. IME sometimes they just die for no apparent reason. I wonder if they have trouble with processed food in their tiny guts. Who knows?
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Hi thank you so much for your response!!! Ill definitely add some more plants and deco, I struggle immensely with numbers so i keep buying things far too small. I wanted to wait until my nitrites came back down before meddling with the scenery but I'll look into getting more in there asap :) I've definitely been very surprised by how well the fish are coping, during the worst of the nitrite spike almost every rummynose was a beautiful deep red colour with green head markings and im starting to think that my water test strips are wildly inacurate because of this. Ive been told that they only get that vibrant in amazing conditions which definitely didnt match up with my water parameters at the time. I'll keep monitoring the cory cats and hope that the rummynoses are just having some swim bladder problems that I can hopefully get fixed soon, glad to know this isnt immediately concerning for them all!!
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I recommend you getting an API master test kit and testing the ammonia. If you nitrites were high then you ammonia probably was too. I use a dedicated turkey baster and pull water from above the substrate and not where the filter blows. You want to make sure they are both 0 for the health of the fish. You may want to add an air stone or sponge filter to the other side to ensure water is being circulated sufficiently to make sure ammonia/etc isn't sitting in one spot. Your filter could be pulling water from one side but not the other. I will also say the anubius should not be potted or put in the sand. It should be tied to decor. If the rhizome is covered it cannot absorb nutrients and will die. Get more plants and caves for the cory cats. They will hide for a while until fully comfortable.
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