What a cleanup crew has to do with it: if you have bought a tank and incorporated too much waste, if you have a tank in which live rock dieoff has produced too much waste, if you need waste 'processed', in company with bacterial action, bristleworms are excellent, and much better than crabs, who can only clean the surfaces. Nassarius clean under the sand, and will not overturn the sandbed in the process. A tank without an adequate CUC is very easily upset. In my tank, I could probably toss a dead smelt in and find it gone in the morning without much hiccup in my nitrate/ammonia situation. A really fat snail checking out under the rockwork without an adequate CUC *that is capable of reaching it* is a problem.
So yes, bristleworms have something to do with the question. This tank is not ready for fish, and needs chemical stability, first, then CUC only for a couple of weeks, then fish.
My question was to ascertain what living inverts may be in the tank at present.