There's been a long debate about whether carbon filters are needed, but I do use them, cleaning once a month and replacing a third every three months. I have cold fresh water, fish. As well as the sponge filters the carbon keeps the water clean and free from smells.
You WOULD however remove the carbon if you were treating the fish with some medicinal chemicals, for disease for example (I tend to steer clear of these anyway), as the carbon would just remove the medical chemical immediately.
As long as you do your weekly water checks, making sure Ammonia and Nitrites are at zero, you'll be fine. Also remember when your nitrates start to rise then it's time for a partial water change (treated beforehand of course). About 30% is what I normally change.
Hope this has helped.
It's 5ml of Aquasafe for ever 10L of tap water you prepare for adding to your tank. I usually have 10L Jerry Can prepared and ready to go at any time. I like the water to be up to room temperature before it goes into the tank.