Both the API ammonia & nitrite (and the nitrate also) kits will give you an accurate reading for both SW & FW.
What size is this tank?
Can we assume it wasn't cycled?
Changing 50% of the water will only lower your ammonia by 50% - and that isn't enough
If you are unable to change 100% of the water, you need to get a product called Ammo-lock which will bind to the ammonia and make it less lethal to your fish. The longer you wait, the more damage being done to the fishes gills by the ammonia.
Please let us know what is happening so we can guide you further to rectify this issue.