If you don't have water conditioner (if you don't know what it is, you don't have it), don't change it straight away out of the tap- tap water has chlorine in it which kills fish.
My personal view is that you should do the following:
Fill a clean bucket with water (not cleaned with soap, clean as in no chemicals at all), and change 2/3 of the water tomorrow afternoon.
Then fill up the bucket again straight away, let it stand for 24hrs again, then change 1/2 of the water. Do this every day for a fortnight or so, moving down to 1/4 of the water in the second week.
Do not wash the filter.
Do not add any fish.
Do take out any dead ones straight away.
Do not feed them today.
Then only feed them once a day, only as much food as they wolf down instantly.
Read what cycling (and the nitrogen cycle) means, using the search at the top of this site- you will probably then understand your problem.
Now you need to very quickly work out what you're doing. If you show willing and effort, you will get all the help in the world. Any immediate questions?
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Any other experienced fish keepers have an opinion on my advice?