stupid_naso
Some folks don't believe having the live rock, live sand AND a wet/dry trickle filter or wet/dry powerfilter is necessary, and thus not a preferred method for biofiltration.
Here's something to think about as an example:
Suppose you have a 100 gallon reef tank, loaded to the hilt with say 125 lbs or premo live rock, and a wonderful live deep sand bed.
The tank's been set up for a year - and has 4 small fish and a ton of corals.
Between all of the surface area of the live rock and sand particles - you'll have most all the bacteria the tank will need to process and convert the ammonia from these fish, and the nitrite too, leaving you with nitrate.
The DSB if done properly should help to take care of reducing your nitrates. The actual test readings may vary - but let's say with that set up - your test's all read zero, or just a hint of nitrate.
What else would you expect the wet/dry to do, besides maybe aerate and move the water somewhat.
You've got the biofiltration being handled by the rock and sandbed.
Now in this same scenerio - hang a wet/dry powerfilter on the back. Now add another, and another, and another ... until you have say a dozen of these powerfilters hanging all around the tank's four walls.
What have you gained ?
You have more biomedia - that was not necessary from the beginning.
The tank is still producing the "same" amounts of ammonia, and now you've got even more biomedia and bacteria to process this "same" level of ammonia.
My thought is that you may be depriving the bacteria in the sandbed and in/on the live rock of the fuels that they need to survive, reproduce and grow. It may be possible to actually have a far less efficient biofiltration system with these additional wet/dry filters - because none of them are fully populated with a healthy population of bacteria - there just isn't enough fuel ( ammonia/nitrite ) to go around. You've "over" filtered this tank to a degree.
The biofiltration system may only be as strong as the amount of fuel feeding it.
You want your greatest numbers of healthy, reproducing bacteria in the rock and sandbed - and by adding the wet/dry to the equation - you may be robbing these bacteria in the tank of the very food they need to live and metabolize the nitrogen compounds.
Wet/dry trickle filters are very good at allowing these beneficial bacteria a "place" to grow, and some feel they may be "too" good, thus not the preferred method - IF you are running the live rock and live DSB as well.
Not so much because the biomedia becomes a so called "nitrate factory" ..... but because you already have adaquate surface area in the rock and sandbed to accomplish what you've set out to do ..... convert nitrogen compounds, ammonia to nitrite, the nitrite to nitrate and hopefully allow the DSB to convert some if not most of the nitrate to nitrogen gas - completing the nitrogen cycle.
Another way to look at it. You've got a group of 6 adult people under one roof, but only enough food a day is available to feed this group of 6, keeping them healthy, happy and active doing some form of work.
Now you add 4 more grown adults to this group, making it a total of 10 people, but still only have the same amount of food to feed the original 6.
You may end up with 10 unhappy, malnoursihed, hungry people, that all become less active, are always hungry, and may become less efficient at doing the work.
This is my view and description of what may happen in a tank such as this, and possibly some other's here as well may agree or disagree.
I'm sure some other folks may be able to explain it in a way that might make more sense too.