There's debate on this. The majority feels that carbon can in fact absorb certain trace nutrients the plants will need...but it's not going to absorb macros like nitrate, phosphate, potassium.
However, that said, regular water changes, combined with not overstocking your tank eliminates any need for constant use of carbon in a
FW tank. Carbon mainly removes odors and discolorations, as well as left over medications.
Some people think that cheap carbon, once used up, will actually leech phosphates back into your water column. However I have yet to see concrete proof of this, and modern carbon is probably better quality than what they had back in the 60's-70's.
Your 29gallon is not overstocked, and if you're keepign up on weekly water changes, it should have nothing more than a damp, earthy scent. If it stinks like fish, you need a big water change and are probably over-feeding (leftover fish meal in the food, decaying=stench of rotting fish).
Also, Carbon is only active for 5-7 days, so it must be constantly replaced if you do want to use it 24/7.
My advice, ditch it...its just an un-necessary expense.
