New 20 gallon high.

If you create a deep sand bed (DSB), you can potentially have a nitrate problem in the future if you don't have a refugium to consume them. Generally you want about 1-2" of sand. The diatoms will reform on the new sand, adding new sand will restart a mini cycle, which is where they will come from.
Diatoms are just a phase in the cycling process. Once they die off, they will release nitrates and that is where people usually see green algae take its place, when you show no ammonia and no nitrite levels, only nitrates. That is when you can do a small water change or add a CUC. I prefer to let the nitrates zero out naturally and let my anaerobic bacteria colonies grow to remove them.