Stu4648 has the concept right, but if I may be permitted, I'll just correct a couple things.
It technically will not take longer to cycle a tank that is well planted initially, as the nitrifying bacteria will appear and reproduce at whatever rate they otherwise would in that aquarium. Each aquarium is a bit different biologically, so there is no set time for this to occur, as water parameters and other things affect the bacteria. But the bacteria will only be present in sufficient numbers to handle the available ammonia/nitrite. So with live plants, there is less ammonia and nitrite, thus fewer bacteria.
Now to the plants. Plants need nitrogen, and most aquatic plants prefer it in the form of ammonium, not nitrate. This is a big difference between aquatic plants and terrestrial. There are a few aquatic plants that do have a preference for nitrate, but the vast majority prefer ammonium. So with live plants in a new tank, provided there are enough of them and some at least are fast-growing, most of the ammonia produced by the respiration of the first fish and the breakdown of organics will be taken up by the plants. Studies have shown that plants actually take up ammonia faster than the Nitrosomonas sp. bacteria; in other words, plants out-compete the bacteria. However, some ammonia gets to the bacteria and as I mentioned above the cycle still establishes.
Ammonia in acidic water changes into ammonium which is basically harmless to fish and plants. The bacteria still take up ammonium the same as ammonia. And plants obviously grab the ammonium. In basic water (pH above 7) the ammonia does not change to ammonium, but the plants still take it up readily, and they have the ability to change it into ammonium in order to utilize it as a nutrient. Plants can also take up ammonia and use it as ammonia for other purposes. The benefit of plants taking up ammonia is that nitrite is not being produced as it is with the Nitrosomonas sp. bacteria.
Only when ammonia/ammonium is no longer available will plants turn to nitrate, and possibly nitrite, though this latter is not well documented. Using nitrate however means the plants have to change it back into ammonium first, and this takes more work (energy) from the plant. Studies have proven very conclusively that when both ammonium and nitrate are present in the water, plants will only take up nitrate when the ammonia/ammonium is no longer sufficient to balance everything else for photosynthesis.
On adding plants first, this has another advantage and that is helping to prevent troublesome algae. If plants are introduced to a new tank from day 1, they will be taking up nutrients. Algae is thus dis-advantaged. A new tank takes time to become biologically stable, and algae frequently appears during this time, but in my experience this is far less of a problem with fast-growing plants from the start. And I do fertilize the tank from day 1, with a complete liquid fertilizer, as the nutrients will obviously be very minimal until organics can accumulate. Sometimes I add fish on the first day, sometimes not, for various reasons.
Last comment about fast-growing plants. Floating plants are ideal for this, because they have the ability of assimilating CO2 from the air rather than having to rely on CO2 in the water. And not only is the CO2 going to be minimal in the water at first, but it takes about four times as long for plants to assimilate CO2 from the water as from air. So floating plants will have a faster start, and thus be using more ammonia/ammonium than lower plants.
Hope this helps, but feel free to question anything and I'll try to answer.
Byron.
It technically will not take longer to cycle a tank that is well planted initially, as the nitrifying bacteria will appear and reproduce at whatever rate they otherwise would in that aquarium. Each aquarium is a bit different biologically, so there is no set time for this to occur, as water parameters and other things affect the bacteria. But the bacteria will only be present in sufficient numbers to handle the available ammonia/nitrite. So with live plants, there is less ammonia and nitrite, thus fewer bacteria.
Now to the plants. Plants need nitrogen, and most aquatic plants prefer it in the form of ammonium, not nitrate. This is a big difference between aquatic plants and terrestrial. There are a few aquatic plants that do have a preference for nitrate, but the vast majority prefer ammonium. So with live plants in a new tank, provided there are enough of them and some at least are fast-growing, most of the ammonia produced by the respiration of the first fish and the breakdown of organics will be taken up by the plants. Studies have shown that plants actually take up ammonia faster than the Nitrosomonas sp. bacteria; in other words, plants out-compete the bacteria. However, some ammonia gets to the bacteria and as I mentioned above the cycle still establishes.
Ammonia in acidic water changes into ammonium which is basically harmless to fish and plants. The bacteria still take up ammonium the same as ammonia. And plants obviously grab the ammonium. In basic water (pH above 7) the ammonia does not change to ammonium, but the plants still take it up readily, and they have the ability to change it into ammonium in order to utilize it as a nutrient. Plants can also take up ammonia and use it as ammonia for other purposes. The benefit of plants taking up ammonia is that nitrite is not being produced as it is with the Nitrosomonas sp. bacteria.
Only when ammonia/ammonium is no longer available will plants turn to nitrate, and possibly nitrite, though this latter is not well documented. Using nitrate however means the plants have to change it back into ammonium first, and this takes more work (energy) from the plant. Studies have proven very conclusively that when both ammonium and nitrate are present in the water, plants will only take up nitrate when the ammonia/ammonium is no longer sufficient to balance everything else for photosynthesis.
On adding plants first, this has another advantage and that is helping to prevent troublesome algae. If plants are introduced to a new tank from day 1, they will be taking up nutrients. Algae is thus dis-advantaged. A new tank takes time to become biologically stable, and algae frequently appears during this time, but in my experience this is far less of a problem with fast-growing plants from the start. And I do fertilize the tank from day 1, with a complete liquid fertilizer, as the nutrients will obviously be very minimal until organics can accumulate. Sometimes I add fish on the first day, sometimes not, for various reasons.
Last comment about fast-growing plants. Floating plants are ideal for this, because they have the ability of assimilating CO2 from the air rather than having to rely on CO2 in the water. And not only is the CO2 going to be minimal in the water at first, but it takes about four times as long for plants to assimilate CO2 from the water as from air. So floating plants will have a faster start, and thus be using more ammonia/ammonium than lower plants.
Hope this helps, but feel free to question anything and I'll try to answer.
Byron.
