If it is a relatively new aquarium or relatively new introduction of the driftwood into the aquarium the charcoal will help filter out some of the tannins. Back in the 80's (when I was a kid) I used charcoal in all my tanks because that was the thing to do... it won't hurt anything.
To answer the original question: Yes, we put the bio media on top of the bag of purigen in the HOB filter on my daughters axolotl tank to keep it from floating. I can not see anything wrong with this arrangement.
I see many new filters (especially low end ones) come with some sort of charcoal with the filter when it is new. This actually does make some sense. The people that use these low end filters are often newb's that setup a new tank and toss in a bunch of fish without any concept of cycling the tank. The charcoal will help buffer the nitrogen compounds a bit while the tank is cycling.
After a tank has cycled and is established I don't see much use for always using charcoal in the filter. I only use charcoal for a particular purpose, like removing tannins from the water or clearing out medication used for a treatment.
To answer the original question: Yes, we put the bio media on top of the bag of purigen in the HOB filter on my daughters axolotl tank to keep it from floating. I can not see anything wrong with this arrangement.
I see many new filters (especially low end ones) come with some sort of charcoal with the filter when it is new. This actually does make some sense. The people that use these low end filters are often newb's that setup a new tank and toss in a bunch of fish without any concept of cycling the tank. The charcoal will help buffer the nitrogen compounds a bit while the tank is cycling.
After a tank has cycled and is established I don't see much use for always using charcoal in the filter. I only use charcoal for a particular purpose, like removing tannins from the water or clearing out medication used for a treatment.
