If you want low tech, meaning low light, you don't need to dose fertilizers. If you want to give the plants a boost, use DIY CO2, but just giving them CO2 for a short time can do more harm than good. Crypts, at least, may die back when you stop the CO2. The best way to do "low tech", in my opinion, is to use a fertile substrate, no fertilizers added to the water, low light, and just wait for the plants to adapt to the conditions and grow at the slow pace that is typical for low tech tanks.
If the tank is a small one, 20 gallons or less, you can use Flourish Excel to give the plants a boost, and continue using it to keep them growing at a steady rate. With larger tanks, Excel can begin to get expensive.