Box filters are pretty useless. Basically the filter is not big enough and does not work fast enough to deal with the waste from the fish. Also if it is a new filter then the bacteria won't have built up in sufficient numbers and you are probably cycling the tank. Check the ammonia and nitrite levels and do some 30-50% water changes if you have any reading. Make sure the new water is free of chlorine and has a similar temperature and PH to the tank.
Decrease the feeding. The more food going into the tank the faster the water will deteriorate and the higher the ammonia levels will get. Fish will be fine getting fed once or twice a week. You can feed everyday if the filters are fully established.
If you have filter floss in the box filter then never change more than 1/2 of it in a fortnight, otherwise you will get rid of most of the beneficial bacteria and cause water quality issues. If you have to clean the filter squeeze the filter floss out in a bucket of tank water and then put it back in the tank.
You can buy sponges for box filters and they last for years and work much better than the white filter floss/filter wool.