What brand of nitrogen are you dosing? Bear in mind that the best source of nitrogen for most plants is ammonia based. But we cannot add ammonia to a tank with fish. If you read the ingredients label for SeaChem Nitrogen it may surprise you.
Ingredients
Amounts per 1 g | |
---|
Total Nitrogen (N) | 1.5% |
Soluble Potash (K2O) | 2% |
Derived from: potassium nitrate, urea
The nitrogen comes from urea but the nitrate comes from the Potassium Nitrate, so you are adding nitrate as a side effect of adding the KNO3 (K = potassium and NO3 + nitrate). If the plants get all the N they need from the fish and the urea, the nitrate will not be used.
You have a pretty heavily planted tank with a smallish fish load. The plants will not be getting enough nitrogen from the ammonia they produce. Most plants prefer ammonium, but again this is not possible to add to stocked tanks. That is why your fert uses urea. Many plants can also use nitrate is insufficient ammonia is available. But this is a much less efficient process because the plants have to convert the nitrate back to ammonia to use it (the reverse of the cycle). When plants are taking up ammonia (as ammonium), they do not produce nitrite or nitrate in the water like the bacteria do. The end result is there should normally be little or no nitrate in a well planted tank and this then may need to be added.
In your case, if you are dosing too many ferts, you are putting the nitrate in. Generally, only a high light co2 added tank needs nitrate additions.
One last note on your testing. If you are using the popular API test kit or one that works similarly, you should be aware of two things. The first is this test is flakey. Bottle number two has a tendency to precipitate in the bottle leaving ingredients in solid form on the bottom. The less often the test is used, them greater the effect. Therefore, when you test you need to do two things with bottle number two. Bang the bottom on a table a few times to dislodge the solids and then, shake like crazy and repeat. You need to get the solids back into solution to get an accurate test.
Two more notes on nitrate tests. First, they are least accurate between 0 and 20 ppm. Second, the way they work is they convert nitrate into nitrite and then read that. So, if one has nitrite in the water when doing a nitrate test, the test results will read higher than the nitrate level really is.
Finally, your tank has no lid, so the water can easily evaporate. Nitrate doesn't evaporate. As the water evaporates the nitrate concentration in the remaining water will increase. You must remove the water with the nitrate in it to lower nitrate levels.