There is a distinct lack of information to go on with here.
Am afraid we will require far more information, hopefully you can provide the following -
1 - Size of tank
2 - Did you cycle the tank and filter?
- If unsure, check the fishless cycling article for more information -
http
/www.fishforums.net/index.php?/topic/421488-cycling-your-new-fresh-water-tank-read-this-first/
3 - What exactly did you do with the tank and filter etc before adding fish?
4 - Is there a filter and heater in the tank?
5 - What fish and how many of each specie did you add?
6 - Do you use dechlorinator/tap safe conditioner?
7 - Can you provide your water parameters?
- Ammonia, nitrite, nitrate and pH
8 - What tst kit do you use?
- i.e API Master Freshwater Kit or paper strip dip kind
9- Can you provide gH and kH?
- If you do not have the test kits for this, you can find this out by looking up your local water authority website.
- Or you can take a sample of your tank water to your LFS and ask them to test for kH, gH and possibly Ammonia, nitrite and nitrate if you do not have a test kit at all. Most LFS will provide this service, sometimes free but mostly for a small fee. Do ask for EXACT test readings, do not just accept an answer of "its fine" from staff, we need the details to be honest.
And finally, can you also provide some idea your usual weekly routine maintenance that you do with water change and cleaning of tank glass etc
A lot of question and sorry if a little overwhelming but the more questions you can answer, the better the picture we have and may provide a more accurate diagnosis of what may be the issue that causing the demise of your fish.