1 death a day

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rob_rob27
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Hi, first time posting so sorry if I miss anything, but it's a weird one.

So the problem, every morning I wake up to 1 dead fish.

Stats: 200L tank
Temp 24 degrees
PH 6.5 -7
NO2 0
NO3 0
General hardiness 40 approx
Carbonate hardness 20 approx
Quarter water change weekly.
Filter cleaned weekly (with fish tank water)

The info: the dead fish in question could be neons, tetras, guppies, or cichlids. At this moment in time I have 2 corydoras, a golden algae eater and a pleco, now all these mentioned fish are fine, the Cory's are breeding and laying eggs weekly and seem to be thriving. However the introduction of any other fish leads to this one death a night. There are no signs of illness on the fish, I have tried sourcing from different shops but the same happens. There's plenty of hiding places in the tank they have plenty of space. I've introduced just a couple of fish at a time e.g just two guppies, and around 20 at a time the same death pattern happens in both.

I have changed fish foods, I have tried increasing and decreasing the amount I feed them, so once a day or twice a day to giving them a fasting day and still wake up to one dead fish.

I do not add any treatments to the water. The tank has been up and running for around a year, full water changes have taken place in this period and start from scratch (Cory's and co go in a smaller tank while this happens)

Does anyone have any suggestions or is one of the fish in there jack the ripper of fish world.

Thank you in advanced
Rob
codheadjohn
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Hi Rob. You say nitrate is zero, but that seems very unlikely. How are you measuring this? What is your total stock list? What filter do you use?

It sounds like what is sometimes referred to as old tank syndrome. This is usually due to high nitrates in the water. Your old fish got used to it as it slowly built up, but new ones get a shock and die off, often leading to disease that infects the whole tank.

Your water sounds very soft for guppies.
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Vale!
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Welcome to AFUK!

Is it just guppies that succumb? If so that may be because the tankwater is way too soft for them - could be contributory. [ah! I've just read your post properly!]

I'm hardly any sort of expert on Golden Algae Eaters, but I believe they can bother/harrass tankmates by trying to graze on their slime coats if they don't have enough algae in the tank to eat. That may be contributory, too.

Are you able to test for ammonia?

Can you lay a pipeline to carry water from wherever-you-are to Milton Keynes? Please?

[ Edit : to acknowledge CHJ's interposed post]
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plankton
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Welcome to the forum. :)
If it had just been guppies, I would have said it's the water, but it isn't.....
The Chinese (golden) Algae Eater (CAE) as @Vale! has said is renowned for causing problems.
I usually have zero nitrates as well - it comes that way from the tap - so that isn't so important, unless you aren't knocking seven bells out of the test kit (if it's liquid). However I suspect (as there's no ammonia reading) that it's strips that you're using which are notoriously very "iffy".
Other questions - how many fish are in this 200l and what are they all (I see you mention cichlids which are grumpy most of the time as well), and what are the dimensions of the tank?
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