Trickle filters as such don't necessarily cause nitrate problems.  That is a myth that you will see popping up a lot, mainly because people don't actually understand what is going on.  Or you will hear people saying bioballs are nitrate factories as if that is something bad. Well, they are in fact nitrate factories producing nitrate since that is what they are meant to do.  But so is any other aerobic surface within a system, including liverock.

No matter what type of aerobic filtration you use, trickle filter, fluidised sand filter, canister filter, live rock, shallow sand bed, etc., the end product is still nitrate.

The issue is what happens after that nitrate is produced.

What you want to happen is the nitrate to then reach a low oxygen region where denitrification can kick in and convert the nitrate to nitrogen gas.  The nitrogen will then eventually leave the water as a gas into the atomosphere.  That is what deep sand beds are predominantly for, to provide that low oxygen area for denitrification to happen.  The reasoning behind trickle filters causing "issues" with nitrate levels is the fact that the nitrate is generated not in close proximity to the denitrifying areas.  Such as the denitrifying regions found within liverock and deep sand beds (at which the surface of both nitrate is formed).  Due to the mass tranfer processes that have to occur from the point of generation within a trickle filter to the denitrying zone within the rock or sand bed, it may be that nitrate levels will be elevated within the water over what they may be without the trickle filter.

Plus, you don't actually need a trickle filter if you have rock and/or sand in a tank.  It wont provide additional surface area for bacterial growth than there is already present within the system that will really be of any help.

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