FYI that nassarius snails aren't prolific eaters of diatoms, they tend to consume more detritus & uneaten food. Plus I noticed yours is a bare bottom tank with minimal sand.
diatoms are usually attributed to silicates which can come with either water source, sand or rocks. They go away once the silicates are consumed over time, provided husbandry is upkept.
If there's an urgent need to get rid of them however, consider getting trochus or astrea snails instead..