yes no clean up crew is required ,
now there are 30+ different types of snails but a good majority of them won't survive in a closed system these snails are basically useless for cleaning . there are about 4 type snails that do fairly well in a closed system and actually do some good ,
1) Cerith
2) Nerite
3) Nassarius
4) Astrea turbo
there are a few others that do good in a closed system but there cleaning ability is almost nill
if you intend on adding snails , most snails should be 1 per gallon or they will most likely starve.
I keep all types of CUC inverts but it's more for the variety of life , yes they will help some , but most of the cleaning is more from proper circulation and your live rock and water changes , your system can control some but its the majority you need to get out
In no means should any invert be held accountable for excess food build up , any excess food will be consumed to a point but like everything all critters poop.
this is when you come across high nitrates and phosphates , and sometimes ammonia
the only way to correct this is manual labor ,
I hope I worded this correctly as I have a hard time typing in a 2X3 box lol