I need help with a wound that appeared on my Apistogramma cacatuoides’ body.

[Cagri]

Hi everyone,
I need help with my Apistogramma cacatuoides. Yesterday I noticed a white, wound-like patch on the body. It doesn’t look cottony like fungus, more like a pale damaged area. Other fish look healthy and behave normally.
I’m also aware that one Otocinclus likely died the same day I added them and I couldn’t find the body despite searching thoroughly. I know a decomposing fish in a relatively small tank can seriously impact water quality and stress other fish, so I’m concerned this might be related.
Fish details
Species: Apistogramma cacatuoides (pair)
Tank mates: 7 ember tetras + 2 Otocinclus (added 3 days ago, one likely died the same day and body not found)
Tank running for about 3 months
Tank
Size: 42 cm cube aquarium (~70 L)
Temperature: increased from 25°C to 27°C three days ago
Filter: Fluval AC50
Decor: catappa leaves, alder cones, Anubias plants
Water & maintenance
Weekly water changes using 50/50 tap water + RO
Two days ago added ~20 L pure RO water
3 days ago added ~5–6 drops of pH minus
No cleaning chemicals, sprays or pesticides used around the tank
Water parameters (JBL ProScan – photo attached)
NO2: 0.25 mg/l
NO3: 18 mg/l
GH: <3 °dH
KH: 6 °dH
pH: 6.6
Chlorine: 0 mg/l
CO2: 35 mg/l
Feeding
Tetra Cichlid Mini Granules
Tropical Soft Line America
Frozen bloodworms once per week
Symptoms & timeline
White wound-like patch noticed yesterday
Fish still active and eating
Other fish show no visible symptoms
Given the recent changes (adding Otocinclus, possible unseen death, RO water addition, pH minus dosing, temperature increase), I’m worried this could be stress-related, bacterial, or due to a temporary water quality issue.
Photos of the fish and tank parameters are attached.
What do you think this could be and what would you recommend as next steps (water change size, quarantine, medication, observation, etc.)?

Btw sorry for poor pic quality.
Thanks in advance.

 

[Cagri]

Update: Following advice I received from another forum, I dissolved and added 4 g of an aquarium salt called "Tropical - Sanital Ketapang" to the tank water.

 

[Mike Wise]

The pictures are not good enough for me to say anything for sure but the last photo of the male appears to show missing scales. Possibly physical damage at some time.

 

[MacZ]

First of all: I doubt it's connected to the los Otocinclus, because Otos always arrive as almost-starved wild caughts. Losses are the norm.

Do I see that right the thing is on both sides?

(JBL ProScan – photo attached)

Please toss that app. Since asoftware update in 2023 the app gives out to many wrong results, suspiciously a lot connected to products they sell an have ads in the app for.

Please get the standard strips with the scale.

Update: Following advice I received from another forum, I dissolved and added 4 g of an aquarium salt called "Tropical - Sanital Ketapang" to the tank water.

That stuff is ... another thing where nobody knows what's exactly in it. Edit: People sell rubbish all the time. You have real Catappa in the tank -> much better. You can get salt (NaCl) without additives for mere pennies. And the Oak bark is just a filler. Don't use the tropical stuff further. Just dose salt the same way if you have to use salt. I would work with humic substances alone.

 

[Cagri]

Balığımın fotoğraflarını onu aldığım evcil hayvan dükkanı sahibine gönderdim. Bana bunun Columnaris olduğunu ve bu aşamada iyileşme şansının çok düşük olduğunu söyledi. Bu hastalığın, çürüyen/bozulmuş yaprak kalıntıları içeren akvaryumlarda akvaryum sıcaklığının artırılmasıyla ortaya çıkabileceğini belirtti. Akvaryum sıcaklığını 23-24°C'ye ayarlamamı tavsiye etti. Ayrıca Sera Baktopur kullanılması gerektiğini, ancak buna rağmen başarılı tedavi şansının hala çok düşük olduğunu söyledi. Gerçekten çok üzgünüm.

 

[Cagri]

Do I see that right the thing is on both sides?

evet her iki tarafta da var.

 

[MacZ]

Okay, if it is on both sides it either got stuck somewhere or it might be a bacterial infection.

 

[Eddy. E.]

Tropical Sanital + Ketapang contains catappa extract, oak extract, salt and vitamin B1, better known as thiamine, which is essential for the nervous system of fish.
Adding one large (!) catappa leaf per 100 litres/26 gallons of water to the aquarium has a significantly stronger effect.
If you also use dried leaves from Anacardium occidentale, better known as cashew nut, you have a very effective anti-virus agent due to its ingredient called pentagalloylglucose (penta-O-galloyl-β-D-glucose), which is very successful against infestation with Ichthyophthirius multifiliis.