Activated carbon might improve the smell a bit because activated carbon is a filter for large molecules.
But really you should be doing a water change. If your ammonia is at 0.25 then the five fish that you have in the tank are being burned. Ammonia burns their gills and causes permanent damage. It's probably quite painful. You want to keep that ammonia concentration down with water changes. The nitrosomonas bacteria that convert ammonia to nitrite will still continue to increase in population even if the ammonia concentration is below 0.25. So... do a water change.
Here's some more info:
http://www.fishkeeping.co.uk/articles_5 ... rticle.htm
You shouldn't have fish in your tank at this time. The nitrite and nitrate are at 0 because the nitrosomonas and nitrospira bacteria haven't converted them yet. Look at this graph:
http://www.fishkeeping.co.uk/images/amm ... iagram.gif
Do you see how at first, ammonia rises? Then the nitrosomonas bacteria grow in number and they convert the ammonia into nitrite. This decreases the ammonia and increases the nitrite. The nitrate is coming from the ammonia. Then the nitrospira bacteria begin to convert nitrite into nitrate.
In other words, the nitrate in your tank comes from nitrite, which comes from ammonia. A tank is considered 'cycled' when the transformation from ammonia to nitrite to nitrate happens fast enough so that the ammonia doesn't have enough time to build up in your water to a concentration high enough to hurt the fish. A fully cycled aquarium has a large amount of ammonia constantly being produced, but as soon as it gets made it gets converted to nitrite, which as soon as it gets made gets converted to nitrate. Each of these three molecules has the same single nitrogen atom, just in different forms. The ammonia form is toxic at 1 ppm, as is nitrite. But nitrate isn't toxic until about 30 to 40 parts per million, which is why you want all of you nitrogen atoms to be in the form of nitrate.
The bacteria are your friends. You want to have a large population of them. Right now, your tank is new and because the bacteria need a long period of time to increase their population, you don't have enough of them yet to handle all the conversion of nitrogen. They live in regions of high surface area, for example the sponge or bioballs in your filter. The more water flow you can get through your sponge, the more bacteria can live there. Don't let the water stop flowing through your filter. It'll get inhabited with bacteria eventually. It'll just take time. You have fish in your tank, so you need to be doing water changes to keep them comfortable. An ammonia concentration of 0.25 means that the tank isn't fully cycled yet and you need to protect your fish until the bacteria can take over for you.
You'll know that your tank is 'fully cycled' when ammonia drops back down to zero, nitrite drops back down to zero, and nitrate steadily increases. Do water changes to keep nitrate below 30 ppm.