Hi Rose. My understanding is that we don't want the plants to have competition from the bacteria which do the ammonia-to-nitrite-to-nitrate conversions. A sponge filter's primary purpose, as you may know, is to provide a ton of surface area to grow these bacteria for the conversion/filtering. Our NPTs have a good balance of many bacterias in the soil and plants do the filtering. The plants will want the ammonia that fish provide as fertilizer. DWalstad's book showed how aquatic plants prefer to uptake ammonia in leaves, over taking up nitrate in roots. I think some people use different filters as needed, like when newly submerged soil is adjusting. My experience is that water movement is good enough, like from a power head. My small tanks, 10 gal and under, sometimes don't even need that. I think my UV filter has been a good investment. I haven't lost LFS fish since I've been putting them with the UV for their first four months. It's possible that the same stores have eradicated disease and parasites in their stock, but I think it's the UV. :laser:
Hope this helps. Best wishes,
M
