Is it good to have nitrates?

[Tropical Tony]

Hi guys,

itested the nitrates on my planted tank before and it was showing somewhere between 10 and 20. Is it good to have a bit of nitrates in there for the plants?

 

[Byron]

No, nitrates are harmful to all fish, and plants cannot use them anyway, assuming this is a low-tech or natural planted tank. High-tech is again different.

Plants need nitrogen as a macro-nutrient, but most all species of aquatic plants prefer ammonium as their source of nitrogen. This is different from terrestrial plants which use nitrate. There is usually sufficient ammonia/ammonium in most fish tanks to supply all the nitrogen the plants need. You would need very intense lighting as well as adequate amounts of all the other nutrients including CO2 before the plants would run out of nitrogen from ammonia/ammonium.

If the ammonia/ammonium should become insufficient, plants masy then take up nitrate. But this is a last resort, because the plants have to use valuable energy to convert the nitrate back into ammonium in order to benefit. There are some studies that seem to suggest plants would use nitrite before nitrate, but they have to convert nitrite back as well, but it may be a less intensive process. Bottom line, keep nitrates as low as possible, zero is ideal, but never above 20 ppm. My tanks are fairly heavily stocked but even so the nitrate has been in the 0-5 ppm range for years now, which may mean zero or 1 or 2, or maybe 5 but certainly no higher. The fish benefit.

 

[Byron]

Just thought of something else that might explain...plants taking up ammonia/ammonium obviously compete with the nitrifying bacteria (AOB, ammonia oxidizing bacteria). But several controlled studies have shown that plants are faster at grabbing the ammonia/ammonium, and they in fact out-compete the AOB. And, plant assimilation of ammonia/ammonium is non-stop, 24/7, day and night. This is why fast growing plants like floating species are termed "ammonia sinks," they can use a lot of it. And the benefit here is that nitrite is not a by-product, so nitrites are less and therefore down the line so are nitrates. Not because the plants are using the nitrates, but because there are fewer being created by the bacteria.

 

[StevenF]

From my own experience aquatic plant will consume nitrate. However ALL PLANTS can process ammonia, urea, faster than nitrate. How do I know? I have a small 1 gallon tank set up for just plants. No animals in it. I use RO water and the only source of nitrogen in it is the potassium nitrate I add. I have also used synthetic urea CO(NH2)2. plant growth is slightly faster with urea. I haven't used Ammonia NH3 but would expect plants to consume it faster than nitrate.

Overall you want some source of nitrogen in your water Ammonia, urea, nitrite, or nitrate for plants to use. For nitrate you only need 5 to 10 ppm of nitrate in most cases You need very fast plant growth to use more than that. Zero nitrate will encourage algae. In a we'll maintained aquarium Ammonia, urea, and nitrite my measure zero leaving only nitrate for the plants to use.