I have been loosing fish to non specific for a long time. Seem to go in batches with a few going then everything seeming ok for while and then another couple. I have always suspected internal parasites but this has just been a hunch.
Internal parasites (leading to death) usually display some typical symptoms
- fish exhibit a good food response, but then just swim aimlessly about as if slightly confused
- fish will intake food, but then spit it back out; they will often continue tasting and spitting, you won’t see the slight roundness through the belly that develops when fish eat a good amount
(fish should rarely show a significant rounding after meals - unless you’re growing out fry - in general, aquarium fish (including some shops) are overfed)
- long trailing (often white) feces
And some more generic symptoms as well
With continued occasional losses, consider whether maintenance might be improved - this is always a good place to start even if water seems fine - a week of daily water changes and substrate vacuuming, filter clean etc, then a couple weeks of 2-3 weekly water changes, gradually returning to standard 50-70% weekly water changes (watch George Farmer change water
😉)
Also increase aeration, gentle splashing of filter return, night time air pump etc for a some weeks (or permanently)
Bacteria can easily build up in substrate and water column (and filter media) with no visual changes or even increased nitrates (should you test your aquarium water), so whenever you experience unexpected fish losses, just increase maintenance
When you get to a point where you want to introduce new fish, I suggest a rescape with thorough substrate cleaning, rinsing and trimming of plants, filter clean etc
VLOG Filipe Oliveira 60L Rescape 
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