Sorry forthe long post but I think you will find it helpfull.
I also use 100% RO water and had the same problem. If Plants will not grow but algae doe well it is a good indications of a nutrient deficiency. And I am not talking about light or CO2. I am mainly talking about the macro nutrients (N,K,Ma,Mg, and S) and the micro nutrients (Cl, Fe, Mn, B, Zn, Cu, Mo, Ni).
The problem is probably your fertilizer. Most fertilizers on the market don't have Calcium (Ca) Nickel (Ni) or any Chloride (yes Chlorine is an important nutrient but not as the toxic gas used to treat water), And typically magnesium (Mg) Copper(Cu), and Zinc(Zn) are all at levels way bellow levels plant need. Others use a form of iron(Fe), Molybdenum(Mo) or sulfate ingredients that don’t last long in the water. To put it simply, fertilizer companies do this to save money knowing that your water will provide what is missing.. For tap water or a over stocked aquarium that is typically the case. Unfortunately RO water has no nutrients in it.
I solved my problem by making my own fertilizer. At the time it was the only solution I had. However now there is one product on the market I can recommend.
GLA micro nutrient . This product is an improved version of CSM+B micro nutrrient CSM was deficient in zinc and the Iron in it unusable in water with a PH above 6.5. The GLA micro product fixes both of those issues. Although If doesn't have a safe chloride salt for (CL). This might be all you need to get your plants growing.
However iff it doesn’t work you will need to dose the macros, a GH booster, Nitrogen(N), Potassium(K), Phosphorous(P)., And Chloride salt for CL. For a GH booster I would recommend a sulfate GH booster such as Seachem Equilibrium or the GH booster sold by GLA. Sulfate GH boosters contain, Calcium(Ca) and magnesium(Mg) sulfate and will address Provide the Ca, Mg, and S plants need. Use the minimum amount necessary to achieve growth unless your fish prefer harder water. 2 degrees (about 40ppm) GH should be all plant need. Just the GH booster GLA micro product might be enough. Note sulfate GH booster must be dosed as a dry powder.
If not N,P,K would also need be dosed.GLA has guidelines for dosing NPK and does sell a GH and NPK supplies. booster. Or you can usea fertilizer calculator,
like this one, to figure out how much to add of each. I currently use potassium nitrate, potassium monophosphate for myNPK. For CL I use ordinary table salt (NaCL or sea salt. Keep the CL dose to same as the iron dose with for GLA micro The GLA dosing guide lines provides 0.05ppm Fe dose.
Also for PH control put a decorative sea shell into the tank (a piece of coral will also work). The shell is primarily calcium carbonate. If the PH drops below 7 shell will start to dissolve and neutralize the acidity and push the PH back up to 7. If the PH is above 7 the shell does nothing. When it does dissolve it will increase GH and possibly LH but for my tank it isn’t enough of an increase to cause problems.
However for tanks with CO2 injection only use the minimum amount necessary for growth (Too much CO2 will also cause the shell dissolve faster than normal). You might not even need a CO2 system. In my tank I use an inverted bottle. The bottle is placed the tank with no air in it and then filled with CO2 gas. The gas will dissolve into the water as needed. For a large bottle it might only needed to be filled once a day. In my tank a small bottle is mounted to my aquarium cover and it is automatically refilled 4 times a day. with the inverted bottle it is impossible to kill your fish with too much CO2. My CO2 usage has dropped dramatically since using the inverted bottle and the plants slowly release bubbles of oxygen when the lights are on. With an inverted bottle a drop checker is not needed and it will not work with an inverted bottle CO2 system.
Also check your tank PH just before the lights turn on and just before the lights turn off at the end of the day. In my tank the PH stays is slightly below 7 in the morning and gradually increase until the lights turn off. I believe this is due to the plants consuming nutrients with the lights on. I believe this is common in many tanks. it can be controlled by dimming the light or turning the light on time. Keeping the PH to no higher than 7.5 prevents the iron from becoming unavailable to plants at higher PH levels.
With the above changes you should not need any special substrates or root tabs or capsules to fertilize the substrate. An inert gravel will work. In my case I use a tumbled Black glass gravel substrate.Also you wouldn’t need to use a water conditioner. RO filters n my experience do a good job of removing toxic free chlorine. from tap water. Also still do weekly water changes. It is impossible to know if all the fertilizer nutrients are consumed by plants. So do a water change to remove any excess. I do a 50% water change once a week and dose my fertilizers at the same time. The sea shell in my aquarium needs to be replaces about once a year.
