Hi,
We spoke briefly on chatroom about this problem.
First off, I think your tank has not be cycled, which basically means getting the tank and filter ready for fish.
http
/www.fishforums.net/index.php?/topic/421488-cycling-your-new-fresh-water-tank-read-this-first/
Have a read of that article.
Basically it explains you need bacteria colonies to deal with the ammonia which fish produces, ammonia is toxic to fish. So it is important information to help you understand what happens in a fish tank.
First thing you will need is a test kit, liquid test kits tend to be more accurate, try to get API FW Master Test kit as this is a popular test kit.
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And posting these results along with some more information will help a great deal.
Tank Size?
Tank Mates?
Water Parameters ? (you can take a sample of water to LFS and ask for specific test readings.)
How long has tank been running?
All these will help us build a picture of whats happening in your tank and hopefully we can then advise whats best thing to do.