I don't keep carbon in my filters as I think it is an unnecessary added expense as you have to keep replacing it regularly. Depending on your filter type, replace the carbon with extra filter wool or ceramic noodles.
It can be interesting at the start to do water tests just before, then just after, you do the water changes. This will give you an idea of whether you are doing the water changes soon enough, or possibly able to leave them a little longer (like your 2nd rather than every week).
An ammonia reading of 0 is good!