This is ammonia rising *before* you put the new fish in? Lets try to seperate the issues out here so we aren't confused.
So you cycled the tank (with fish?) and then your ammonia is zero, but then ever since the ammonia has been going up after water changes? However if the ammonia is 0.5 before a 50% water change it could only be 0.25 at best after, so how much of the water are you changing?
Could you go through it all in excruciating detail for us please? Don't leave anything out.
My first thought is that there is either a problem with your filter or your filter is insufficiently powerful, however we need to have more information.
135 litre (35 US gallons) 6 Black Widow Tetra; 3 Mollies; 3 Bronze Cory; 1 Pygmy Cory; 6 Gold Barbs
27 litre (7 US gallons) 6 Guppies.