Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Slow release fertilizer in a form that is placed under the substrate might work, even if it is designed for house or garden plants. I have heard one form of Jobes spikes (the skinny little house plant ones) can be broken into 2-3 pieces and placed sparingly under the substrate. I think it was the one for ferns.
Osmocote product line is another possibility. The pellets can be frozen in small ice cubes and the ice cubes pushed into the substrate. The ice melts, and the osmocote is slow release fertilizer. Osmocote Plus is the one I see mentioned most often. If you have a scale that weighs small amounts figure 5 grams of osmocote per square foot of substrate.
Tablets designed for ponds and aquariums will also work. The pond tablets are pretty big.
Floating plants get their nutrients from the water and the air (CO2). Substrate fertilizers will not help them.
Land plants seem to do just fine with nitrates. Ammonia on the land, in the soil is converted very quickly to nitrate, so the plants have evolved to be reasonably efficient at using it. Ammonia is also still a fertilizer to them, and they are pretty efficient at using it, too.
Aquatic plants seem to use ammonia most efficiently, but will also use nitrite and nitrate just fine. In a cycled tank it is a race between the bacteria and plants, who gets the ammonia, then the plants remove any nitrite they find (very rare in a well cycled tank) then nitrate. Since the ammonia is used up pretty fast, and any nitrite does not show up in the water (it stays in the biofilm where the bacteria use it up), the nitrate test is the only one that shows if the system as a whole has enough nitrogen.
Since your NO3 test is showing the level is so low I would be taking steps to raise it.
a) Skip water changes, or do smaller ones.
b) Dose nitrogen in any of several natural forms: Add more fish food, for example. I find this a pretty expensive way to add N, though. The fish overeat, and the cost of N from fish food is really high.
c) Dose nitrogen in any fertilizer form: Slow release fits the concept of low tech better than daily dosing. You could start adding N from KNO3, if you wanted. But that needs to be done more frequently. Easy enough to do, and the cheapest form of N.
If you are also seeing phosphate deficiency, then the same solutions work for that, too. Fish food contains phosphate, and fertilizers (KH2PO4 for example) or slow release ferts. Read the label.