Half my fish dead, rest looking unwell

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Out of interest, what was the fish stocking level like (before the incident) in your 125 aquarium? Also, did you happen to deep clean the substrate during the latest maintenance?

You say that it is usually a bit cloudy after you do maintenance, but is the water crystal clear normally? i.e. not dull or slightly milky?

I must say, it is a bit odd and strange for something to go wrong to that degree so quickly. Wondering if it was a combination of things which was caused by the cleaning and a tank 'on the edge' of the load etc.

Send a few pictures if you want - might help us get a view first-hand on what might have gone on.
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It does sound like something nasty has got in the tank, and, with the filter having been cleaned (maybe over-cleaned) the two couldn't cope with what was going on.
The water supply (as black ghost has said) would be the first thing I looked at after doing a parameter check.
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It look like all the gunk has come from the phosphate remover as shown in the photo.

No idea what caused it but i wont be using it again.
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Sorry to hear about your losses, but I'm glad to learn that your tank is recovering.
Juwel Trigon 190:

5 Bronze Cory
1 Panda Cory
1 male BN Plec
15 Tiger barbs
5 Golden Wonder Panchax
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black ghost wrote: Tue Oct 31, 2023 20:43 pm
Don’t use PolyFilter (or carbon) in the tank. Keep it for emergencies. Replace it in the filter with sponge or other inert media.
Hi Black ghost, i am interested in your reasons for eliminating the use of polyfilter. Are you saying its is better not to use it in the filters at all and to use other sponges in there instead? Why would that be? and what would you use instead?
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I’m biased towards keeping big fish. :)

I filter all my water through PolyFilter for a few days before it goes into the tank. This way it lasts about a year. If I use it in a tank with big fish it only lasts a few weeks. But in a tank of small fish it will last a lot longer so feel free to use it.

Note the exact spelling of PolyFilter though. It’s a specially impregnated wonder-foam. Other foams called polyfilter are available but those are just untreated foams, cashing in on the name. If it’s not in Arcadia, Underworld or Poly-Bio-Marine packaging it’s not the real deal.

Carbon has microscopic pores that give it a huge surface area for absorption/adsorption of impurities, but in use the pores quickly get blocked, which prevents it from working and it just becomes an ‘ordinary’ filter medium.
I don't keep fish, I keep water. Water keeps fish.
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Thanks Black ghost. I find its important and helpful to understand someones justification to use or not to use something.
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