What might cause a Celestial Pearl Danio to go off their food?

Question

Overview

One of my celestial pearl danios (CPD) has gone off their food and is skinny. Whilst it might be too late for him, I want to save the other 5 CPDs in the tank. Any ideas?

Background and recent tank history

Last week my Betta died at 2 years old. Initially I thought something may be wrong with the tank since I thought they were a 3-5 year life expectancy, but the local pet store guy, who is an avid hobbyist, said 2-3 years is becoming more and more common due to intense inbreeding in recent years.

Since the danio is now sick, I am not entirely convinced it was only the betta's genetics to blame.

Sick danio symptoms

One CPD has now gone off their food, stays still when the others leave him alone, and has a flat belly compared to the other male, and the dorsal fin and pectoral fin are held close to the body. I would describe their movement as erratic, especially when food is in the water.

Another has had an arched back and "bobs", which sounds swim bladder related until you consider she has had this since maturing to adulthood months ago, indicating a genetic developmental defect.

Besides the skinny one, all exhibit very typical behaviour.

Parameters

This is my most recent log entry from the weekend. These parameters are how the tank has been for months. A cup or two of water changes is made every day, and RO:hard tap water 50:50 is used to replace it.

• 25 degrees C

• Air bubbles through sponge filter 24/7

• 10-25 ppm Nitrites

• 0 ppm Nitrate

• 0 ppm Ammonia

• >4 <7 GH ppm.

• 6-15ppm KH

• 7-7.6 pH

• 0 ppm Cl

Tank contents

• 8.5 gallon tank
• 6 x CPD (2 males 4 females) (1 curved back and floating issues, 1 retracted dorsal and anal fin)
• 1 x Horned Nerite
• Some small snails
• Monte carlos lawn (good health)
• Tiger lotus (green, room for improvement health)
• Anubias barteri (some browning tips, also new growth)
• Egeria densa (string algae infection, a couple of new growth areas)
• Water lettuce (good health)
• Duckweed (depleting, but there has been a lot disturbance)
• Frogbit (many browning leaves, but generally good health)
• Marimo balls x 4 (good health, there was a dead patch which is recovering rapidly)
• Java fern (No large leaves new growth going well)
• Bacopa (declining health, yellowing leaves)
• Vallisneria spiralis (good health)

Recent upsets to the tank, and failed Amano shrimp introductions

This may be by the by, but it might be one of those niche things someone else may have experienced. Since the betta died I have gone through the tank thoroughly removing anything that could be an issue. I imagine this would have been very distressing to the fish.

During the declutter I found a couple of rocks that looked to be rusting and were potentially leaching iron into the water. You cannot see the iron vein from the outside, it must be on the inside of the porous surface of the stone. Over the last year, I have tried introducing Amano shrimp to the tank. Each time the betta harassed them and none survived 24 hours. However, I tried another before this declutter, but after the betta died, and that shrimp too did not survive 24 hours. I suspect these dodgy rocks are to blame.

Treatment so far

I am at a bit of a loss since the symptoms are a bit vague, and the tank is otherwise healthy. But CPDs are supposed to be bullet proof and these are barely 2 years old.

With that in mind, I decided to add in some boiled Alder cones and treat with methylene blue and acriflavine at the recommended dose.

I posted this previously to reddit and whilst it did not yield much useful information, someone raised the question of parasite treatment as a possible solution, which could have been introduced by the recent Amano addition.

Any ideas for the next steps are very welcome.

I intend to not treat further if no other fish become sick, and just assume the stress of the declutter shocked the fish fatally.