i was told that the bioballs should be removed gradually from the wet/dry filter. is this true?
If you have enough surface area in the LR
and sandbed for nitrification, then yes I would remove the bioballs, wet/dry trickle filters are nitrate sinks. Depending on your bioload, if you remove them it should be done slowly over the course of 2-3 weeks.
Here is an article about the basic principle...>>The Berlin Method of Filtration