Need help before more fish die

Anyone know what this is?(Cory with fungus looking area behind head and stubby fins) I figured it was fin rot but am now worried columnaris. Sadly he died this evening but am including pics of the other fish in case someone sees something I don’t.

No other fish APPEAR to be sick to me, but I’m worried about them. Should I treat whole tank? Worried about beneficial bacteria and plants. Have never had sick fish. I have ich x, maracyn 2 and expired Paraguad and kanaplex. I ordered maracyn but it will take a few days to get here.

I can’t see anything wrong with other fish, but tetra fins are thin and hard to tell if they are fraying, Gourami looks fine and other cories look fine.


Tank
What is the water volume of the tank? 40G
How long has the tank been running? 5 years, but filter (fluval 406) went out so I hooked up my old 206 and it caused nitrate to go off chart. I have been doing water changes (50% twice) and it’s now down to 40ppm
Does it have a filter? Fluval 206
Does it have a heater? Yes
What is the water temperature? 77-78
What is the entire stocking of this tank? (Please list all fish and inverts.) 1 Congo tetra, 1 lemon tetra, 1 dwarf blue gourami, now 5 Cory, 11 Rio tetra and somewhere, maybe.. 6 Amano shrimp

Maintenance
How often do you change the water? Every other week to 1 month normally, every 2-3 days now to get nitrate back down.
How much of the water do you change? Usually 8-10 gallons
What do you use to treat your water? Seachem prime
Do you vacuum the substrate or just the water? Substrate IF there is debris

*Parameters - Very Important
Did you cycle your tank before adding fish? Yes
What do you use to test the water? API master kit (test strips in between because I added more fish (rios) just to make sure ammonia doesn’t spike
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.
Ammonia: 0
Nitrite: 0
Nitrate: 40ppm
pH: 7.8? Hard to tell.. yes I have liquid rock and am using spring water to slowly lower it

Feeding
How often do you feed your fish? Sometimes daily, sometimes skipping several days
How much do you feed your fish? Enough that they will eat..
What brand of food do you feed your fish? Varies, lately small pinch of bug bites and hikari nano and 2-3 catfish pellets
Do you feed frozen? Rarely bloodworms
Do you feed freeze-dried foods? Not in the past 2 or so years

Illness & Symptoms
How long have you had this fish? 2-3 weeks
How long ago did you first notice these symptoms? Today when I found it against the sponge filter
In a few words, can you explain the symptoms? Stuck to sponge filter, thought it was dead but it swam away so I caught it and tried treating with Maracyn 2.. died several hours later
Have you started any treatment for the illness? Maracyn 2
Was your fish physically ill or injured upon purchase? Not that I could tell
How has its behavior and appearance changed, if at all? The cories hide

Explain your emergency situation in detail.
(Please give a clear explanation of what is going on, include details from the beginning of the illness leading up to now)

 

The filter change will not cause nitrates to go up. The new fish caused and increase in ammonia which is ultimately converted to nitrate. I'm not saying that your ammonia spiked, but rather that it increased and your biological filter (bacteria) was able to handle it, thereby increasing nitrate. 50% weekly water change is typical. You need to increase the frequency and amount of changes. Fish will prefer stable conditions.

I don't know what killed your cory, but if the water is not optimal, any disease becomes more possible due to the increased stress on the fish.

You only had it for 2 weeks so it may have been ill when you got it. The change in water conditions that new fish experience can be stressfull (temp, pH KH, GH, etc) as can being moved to a new environment. It's important to acclimate them and put them in pristine water if possible.

Best practice is to quarantine all new fish for several weeks. This will help prevent introducing disease to your established tank. It will also help determine if the fish was diseased when you got it. If it does turn out to be diseased in some way you can then treat it without having to treat your entire established tank.

 

For the nitrate, normally I would agree, however I did a 50% WC and it didn’t budge. I got new filter hoses and it cut in half with a 50% change like it was supposed to.. I had read it on another forum, can’t remember where.

I’m mainly worried about columnaris.

 

From my knowledge, catfish are normally not that susceptible to columnaris, and it usually affects livebearers and sometimes tetras and relatives. However your attached image of the cory does look very suspicious. I would posit that this cory was already diseased and considerably weakened when you got it. It appears to have also suffered from advanced fin rot. In my experience cories only tend to end up in filters if they are either young and can't escape its suction or if they are too weak to due to disease - cories with some sort of gill inflammation or fluid buildup (essentially, their version of pneumonia) will visit the higher levels of the tank much more frequently as their breathing is too shallow for them to get enough oxygen at the bottom level, potentially leading to this situation. Considering all of this, your cory may have had one of a range of bacterial diseases.

Your other fish appear to be healthy, but I'd go ahead and administer a course of treatment, just to be on the safe side. Make sure to remove any carbon from the filter before starting the treatment. Also make sure that any medications you are using aren't harmful to the beneficial bacteria in your tank, as this can lead to more harm than good in the long run.

 

From my knowledge, catfish are normally not that susceptible to columnaris, and it usually affects livebearers and sometimes tetras and relatives. However your attached image of the cory does look very suspicious. I would posit that this cory was already diseased and considerably weakened when you got it. It appears to have also suffered from advanced fin rot. In my experience cories only tend to end up in filters if they are either young and can't escape its suction or if they are too weak to due to disease - cories with some sort of gill inflammation or fluid buildup (essentially, their version of pneumonia) will visit the higher levels of the tank much more frequently as their breathing is too shallow for them to get enough oxygen at the bottom level, potentially leading to this situation. Considering all of this, your cory may have had one of a range of bacterial diseases.

Your other fish appear to be healthy, but I'd go ahead and administer a course of treatment, just to be on the safe side. Make sure to remove any carbon from the filter before starting the treatment. Also make sure that any medications you are using aren't harmful to the beneficial bacteria in your tank, as this can lead to more harm than good in the long run.

So should I treat for bacterial? Or something else? I read this article and have the meds (neighbor said I could use her maracyn because I can’t find it locally.

 

Update.. no more fish appear to be sick.