In my garden, I line the outside with marigolds (not the crab safe ones- they’re in a different garden). The marigolds are a nematode deterrent because they don’t like the chemicals in the roots or something. Without the marigolds, the nematode population in my garden would explode and they would kill our fruit and veg.

Typically in the tank, the only time they’ll affect the crab is if it has shell rot, is injured, or is dead.

Nematodes prefer moist soil so if there is excess moisture (from condensation and/plants) they’re gonna dig it.

Are they a risk to people? Only if you aren’t washing your hands after doing stuff in the tank and then eating

Generally, are they a risk to crabs? No.

Do they benefit the tank? Yes and no. Yes because they breakdown bacteria and microbes (sort of neutralising things, keeping it stable)... no, because if fungus has laid dormant in the sub (like in the EE) they can redistribute it and sort of wake it up. Fungal blooms seemingly out of nowhere can be associated with higher moisture content in the sub and nematodes.

I wouldn’t say isopods eat nematodes, I’d say they outcompete they for food. Having isopods will definitely help with nematode numbers in the top few inches of sub, however the nematodes will rule the depths (in a moist tank).

A false bottom and all the amendments will help with drainage avoiding a potential moisture buildup.

Do people need to take action against nematodes? No, but they do need to observe for any changes in moisture and/or a noticeable increase in sightings.

Random suggestion for extra sightings: Do a flood test and also rule out plants being a possible cause.