Half my fish dead, rest looking unwell

[agb123456]

Hi All,

I have an established 125L planted community tank been running for 4 or 5 years, it has an internal filter and ive since also added a fluval 207, change approx 30% of the water every week and everything was fairly stable until a few days ago.

During the last water change i also cleaned the fluval filter, rinsed all the filter media in removed tank water and placed back in filter and replaced the phosphate removal granules and carbon (I do this every 3 months or so). Immediately following the water change and filter clean the water was a little bit cloudy as is often the case but normally clears after an hour or so.

The following morning there was a very strong foul smell from the tank and the water was cloudy such that you could barely see into the tank. I opened up the filter and the top of the filter where the phosphate removal and carbon is held contained a pale brown sludgy material that seemed to be the cause of the smell. Stripped down and cleaned the filter and replaced approx 75% of the water. All my amano shrimp were dead and around 15 or so of my fish.

Water is now starting to clear but the remaining six fish i have are slow and not moving too much.

Any ideas what happened here?

Thanks

[fr499y]

too many changes too quickly causing the cycle to stall.
Don't use carbon unless you are trying to remove medication from the water, it's a waste of time and money.
Do you really need the phosphate remover? have you checked your phosphate levels to see if its actually needed?

[agb123456]

Thank you for the tips.

Ive ran the phosphate for about 18 months now as phopshate levels were a bit high back then, cant remember the actual readings and its not something i test for regularly but think i will stop with the phosphate remover and start checking regularly. Will also remove the carbon.

Ive done the weekly 30% water change for years and maintained the nitrate levels at around 10ppm. What do you think a more reasonable change frequency should be? I never realised that things could go so wrong so quickly.

[LookoutTrout]

So it was all the normal routine with no recent changes?
A 207 filter is pretty idiot proof, I've had a few fish die off but never gunk and a bad smell so I have no idea what happened.

[Lo1]

Really sorry to hear about your losses.

A couple of thoughts, the pale brown sludge most likely came from the phosphate removal granules.

Carbon is very good at removal! It remove both good and bad nutrients from the water, so if you continuously use it, it’s gonna take the good nutrients out of the water which will lead to long-term problems.

Personally I wouldn’t use either of these, continue with your level of water changing each week. And you will keep your phosphorus levels down.

If you feel you need to use something I would cut I piece of this:



And place it in your internal filter.

I always keep a couple of boxes of polyfilter.

[black ghost]

too many changes too quickly causing the cycle to stall.

Ignore this, it’s not a thing.

The carbon and granules all get covered in bacteria just like the other media. You’ve probably removed too much bacteria by throwing them out.
Carbon is unnecessary 99% of the time. I would swap it for regular media.
Nothing wrong with your water change regime.

You also might have got a bad batch of tapwater (any roadworks on route?). Carbon might help there but if it’s not new it won’t have much room for more absorption so won’t do it’s job. Keep carbon in case you need it (PolyFilter is even better) and put it in new if you do.

[fr499y]

too many changes too quickly causing the cycle to stall.

Ignore this, it’s not a thing.

What I meant was removing too much bacteria causing a spike.

[LookoutTrout]

You say planted, are you adding CO₂?
I'd guess not with the phosphate remover but you might be.

[agb123456]

Thanks all,

I think Lo1 is right when they said the gunk was from the phosphate granules, there seemed to be more gunk in that section of the filter. I will remove the carbon and phosphate granules from the system and try the polyfilter suggested.

My tank is a juwel rio 125 system that has its own internal filter system in addition to the fluval 207 ive added and i didnt touch the internal filter during this clean so even though ive removed bacteria when cleaning the 207 would there be enough bacteria in the internal filter to continue to treat the tank?

This happened really quickly overnight and the smell was such that it was noticeable when you entered the room. The remaining fish seem to be looking a little bit better now.

Keen in identifying the cause now so i dont repeat the same mistakes again in the future.

[black ghost]

Did you check for ammonia, nitrite etc? I don’t understand the gunk thing, so I don’t know what happened. Whether it has killed bacteria or not. But they can double their numbers in a day….

Don’t use PolyFilter (or carbon) in the tank. Keep it for emergencies. Replace it in the filter with sponge or other inert media.