Today I bought a new API nitrate test kit, primarily because I had a plastic envelope filled with what I thought was KNO3, and my working supply of KNO3 is near zero. With that kit I measured the nitrate in my tank water as it is today, between 10 and 20 ppm. I canít distinguish the colors any closer than that. I measured my tap water, which should have near zero nitrates, and it is zero. Then I mixed a tiny bit of my supposed KNO3 in a glass of tap water, and measured that, getting over 60 ppm of nitrate, so it really is KNO3.
Iím posting this to show how I decided to fertilize my tank, after a lot of thought triggered by Marcelís threads on the subject. This is the first time I have done any serious thinking on this subject, so I thank Marcel for ďturning me onĒ with his posts.
I have a 65 gallon tank, which contains about 56 gallons of water, due to the substrate taking up part of the volume. Based on reading some of Tom Barrís articles in his Barr Report, on the internet, plus Edwardís PPS Pro recommendations, and Marcelís test results, I want to have about 15 ppm of nitrate in my tank water. This is because the uptake of nitrates by the plants is partly determined by the concentration of nitrate in the water, and my desire to have enough to discourage blue green algae (BGA). I donít want the concentration to increase between my water changes.
The three references above say the daily uptake of nitrate for a heavily planted, high light, high CO2 tank, with good nutrient levels, is from .1 to .7 ppm. My tank is moderately heavy planted, perhaps half of ďheavily plantedĒ, so that will reduce the uptake of nitrates to about .05 to .4 ppm. I doníít use CO2, but I do use Metricide for a carbon source. And, my light intensity is low medium, around 40-45 PAR, with my Finnex Planted Plus located about 23 inches above the substrate. This should reduce the uptake to about 1/3 to 1/4 of what it would be with high light and CO2. (Iím assuming my plant growth rates will be between 1/3 and 1/4 of what they would be with high light and CO2). Based on that my daily uptake of nitrates should be about .01 to about .15 ppm. I will assume that .1 ppm is the best estimate.
In 7 days, my 15 ppm of nitrate should drop to about 15 - (7 x .1) or to about 14 ppm. Clearly, I wonít need to dose nitrates more often than weekly. Assuming I do 50% water changes every 2 weeks, as I want to, I really donít need to dose any nitrates during that 2 weeks, and the nitrate level should only drop to 13 ppm, and even if the uptake is .7 per day, in 2 weeks the level should only drop to about 5 ppm, which is acceptable to me, but unlikely to actually go that low.
My fertilizing plan will be:
Dose enough KNO3 after each 50% water change to raise the diluted tank water nitrate concentration back to about 15 ppm.
The next most critical fertilizer element is potassium. If I have 15 ppm of nitrate, I also have 9.5 ppm of potassium, the ďKĒ part of KNO3. But, I want at least 15 ppm of potassium, primarily to prevent it from being the limit on plant growth. I will need another source or sources of potassium. My phosphorous will come from KH2PO4, which contains 40% as much potassium as phosphate (PO4). That will be one source, and, for now KCl will be the other source.
I want 3 ppm of phosphate in the tank water, primarily to try to avoid Green Spot Algae (GSA), which my experience shows will not appear if I have relatively high phosphate. The plants only need around 1 ppm, so 3 ppm should be enough, and should not be a toxic level. If I have 3 ppm of phosphate I will have .4 times that added potassium, or about 1 ppm. This will leave me 4.5 ppm short of the 15 ppm I want, so I will use KCl for that.
Daily uptake of potassium and phosphate should be approximately proportional to the daily uptake of nitrate and the starting concentration of phosphate or potassium compared to that of nitrate. So, if I donít need additional dosages between water changes of nitrates, I also wonít need additional dosages of the other nutrients.
Trace elements are going to come from dosing Millerís Microplex, and chelated iron just because I have lots of them. I will dose the Millerís Microplex to get .5 ppm of iron, and add enough chelated iron to get to about 1 ppm total iron.
Using the fertilizer calculator at Aquarium Nutrient Calculator
my post water change dosages will be:
KNO3 - 10 ppm NO3 6.5 ppm K - 11/16 tsp
KH2PO4 - 2 ppm PO4 - .7 ppm K - 3/32 tsp
KCl - 3 ppm K - 7/32 tsp
Millerís Microplex - .33 ppm Fe - 5/16 tsp
Chelated Iron (10% I think) - .33 ppm Fe - 1/8 tsp
Some of these dosages are going to be very hard to measure at all accurately, so I will use my 16 ounce dosing bottles, which have a 1 ounce dosage chamber. One bottle will have the macro nutrients and the other the micro nutrients. To figure out how much of each chemical to put in the bottles, I will put 16 times the one dosage amount, and fill the bottle with 16 ounces of tap water. These mixes should last me 32 weeks - 8 months!! For that to work I think I will have to add some Metricide to the micros bottle to stop any microscopic life from growing.
I welcome any suggestions about changes I should make.