The demand is differant on every tank so things vary based on the plants, lighting, the entire bioload in general just like every other thing related to planted tanks. My choice was to cap with Flourite. It's got great CEC, it's heavier than other substrates like EcoComplete, turface etc. Sand caps can have issues over soil not allowing enough gas to exchange. I've read several threads regarding problems with sand caps.
My soil tank is still growing plants after 1 year 9 months without adding anything but new water and CSM+B on a regular schedule (ha ha 'schedule' really hit or miss on that too) About 1 to 1 1/2" soil and a 1" cap. HTH
My soil tank is still growing plants after 1 year 9 months without adding anything but new water and CSM+B on a regular schedule (ha ha 'schedule' really hit or miss on that too) About 1 to 1 1/2" soil and a 1" cap. HTH