i cant do a big water change yet as the tank is only about 6 weeks old i have done a 20% change last week and cleaned out the foam filters and media and carbon, when i done the change.
I am not sure who told you that you can't do big water changes on your tank because it is new, but they are wrong. You can do big water changes on a tank anytime you want as long as the new water is free of chlorine and has a similar temperature and PH to the tank. It can cause issues during the first few weeks by diluting the ammonia in the water and reducing the food for the bacteria to grow on. However, most people put plenty of food in the tank and that gets the levels back up pretty quick.
Your problem probably arose because you cleaned out the filter materials last week. Only wash filter materials out in a bucket of tankwater and never replace more than half the filter material at any one time. In fact the only thing you need to replace in the filter is carbon and that should be done about once a month for normal carbon, and once every two months for activated carbon.
If you have iron plant fertiliser in the tank then don't use carbon at all because it will remove the iron from the water. Likewise if you have to treat the fish with medication take the carbon out and throw it away. When you have finished medicating the tank and the fish are better, do a water change and put some new carbon in the filter.