Well, depends on the plants you are growing. Take for instance heavy root feeders like Swords. They will benefit trmendously from substrate fertilization. But plants like rotalas, try uprooting some. You will notice the root system is very thin. They can feed from the substrate but mainly the roots anchor them down. They will find most of their nutrients from the water column. Since we don't know if you use co2 or how much light you are using, hard to say where the spurts come from. Could be due to a water change when you replace the tabs. Now that you have clarified ur initial question, i have no idea about the difference between O and O+.