Water levels and looking fish.

Not the best but he's turned a kind of cream/silver!

Rosy barbs come in a gold form that can look like the fish in the picture when they display. But it could be a disease like microsporidian. This causes the muscle tissue to go white but it takes a few weeks to happen.

can you post a short video of the fish?
put it on youtube and post the link here if you can't post it directly here.

how much water do you change during water changes?
do you use mains/ tap water?
do the fish die within 24-48 hours of you doing a water change?

what symptoms do the fish show before they die?

The water changes are done as close to weekly as I can get, some a day or too late. I do around 50 % and use tap water. I use a bucket that is kept for the fish, and add tap safe to the bucket before the adding to the tank. The deaths don't seem to be linked to the water changes.

The fish seem to get a bit lethargic and sit on the top for a day or so and then spend a day sat at the bottom and die that night. The cory just slowed down a lot, breathing looked quite heavy and died in the evening.

About to do the school run. Will try to post a video a bit later.

I think there is a protozoan infection (spironucleus?) in the tank that is infecting the fish's internal organs and causing organ failure. Metronidazole is something I don't normally recommend using because it was designed for people and improper use can lead to drug resistant bacteria. But this might be one time it could be used.

There are several fish medications containing Metronidazole and they work best when fed to the fish. Depending on what country you live in, will determine what is available to you. But contact your local petshop and see what they have containing Metronidazole and try to get it in a food rather than a tablet you put in the water. The medication wipes out filter bacteria so medicated food is better for the filter and it works faster.

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If you can't get Metronidazole then a broad spectrum medication that treats protozoan and bacterial infections should work but might wipe out the filter bacteria too.

Make sure it is suitable for scaleless fish like catfish. If you can't find a medication specifically for catfish and loaches then use a normal broad spectrum medication at half strength.
*NB* Metronidazole should be used at full strength (recommended dose) for all fishes.

If possible isolate the sick rosy barb and treat it separately and see if it responds. If it does then treat the main tank. If it doesn't work then at least you haven't wiped out the filter on the main tank.

It's worth trying! I'll do some research on it and see what I can find. We're in England...

I'll see what I can do to seperate the rosys, the only other tank I have is my daughter's and don't want to infect her fish.... I'll see what I can find! Thanks so much for your help!

You should be able to get it in the UK. Lots of members on this forum are from the UK and some of them might know where you can get some. Perhaps start a thread asking if anyone knows where you can get Metronidazole in the UK.
I have added a link to seachem and they sell it. This is the sort of thing you want. Ignore the website where it says it treats viral, it doesn't treat viruses.
https://seachem.com/metroplex.php

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Are your daughter's fish ok, and what fish does she have?
If her fish are ok then it means the problem is with your fish and tank and we can rule out water supply and general tank maintenance

She has an 80l with about 10-15 guppies and a few random fish from her uncle. Hers are all fine, we've had it set up about a year all are in excellent condition!
For a 5yr old, she's a pretty good fish keeper
I've ordered some seachem. So hopefully that'll work!
Thanks for all your help. Will update in a couple of days!

Quick update, I got the treatment Wednesday afternoon, i gave half a dose in the afternoon and half the next morning, no more dead fish and all seem a lot happier apart from my cory, could this be because he is now on his own?

After all this one of my sharks is looking very skinny, but he is eating and active so not sure why he is so skinny, it's almost like he is growing in length but not width....

Anyway, thanks so much for everyone's advice. They all seem much better!

Corydoras are social schooling fishes that like to be in groups. If you have one on its own it will hide and be more stressed out. However, because there is a problem in the tank, I would advise against adding any new fish for a couple of weeks after everyone has settled down and you haven't lost any more fish.

Your skinny shark could have intestinal worms. There are threadworms (round worms) and tapeworms that affect fish. Some threadworms can be seen as little red hairs sticking out the fish's butt. Most of the others you don't see.
Any fish deworming product should work on threadworms but you need Praziquantel to treat tapeworm.