...If they get more then 4-5 inches long you are overfeeding the tank....
I'm not sure if I completely agree with that due to their ability to consume detritus and waste (as well as waste which has been re-digested several times over) in the tank. I also suspect they can thrive on minute levels of nutrients. Also there are just some species which get large.
I kept a small fishless reef tank which housed only trumpet coral frags (no CUC or purchased inverts) and in 20 months never fed the tank, yet it had pods and bristleworms all the same.
I suspect the pods consumed film algae and the bristleworms consumed their waste. The largest of the bristleworms was about 3"; not quite 4-5" but again I never fed the tank. I do agree that overfeeding will result in a larger bristleworm population though.
In my 65gal reef tank I keep three small fish (2.5"-3") and for about the last 3 years have fed the tank very sparingly. I provide them this small pinch of pellets every 3 days:
The fish eat the majority of these pellets before they fall to the sandbed and what they miss, my hermit crabs, two nassarius snails, and my serpent seastar consume.
And yet I have one of these in my tank:
The entire body of this large-species bristleworm appears to be several times greater than my largest fish (a royal gramma). I catch rare glimpses of this giant a few times a year (usually during the very early morning hours).
I don't think it is a product of overfeeding because as mentioned I feed only a small quantity every three days and the fish and CUC eat all of it before the lights go out (this thing hates the light).
In the bottom left corner of the first photo is an amphipod burrow filled with its waste and other detritus. I've seen the small crustaceans sort through and re-consume that matter over and over again. I suspect that this large bristleworm (and the smaller bristleworms in my unfed frag tank) patrol the sandbed for micro-particulate matter such as this (digested matter from fish and pods, as well as detritus). And again, I do believe overfeeding will result in a larger
population of bristleworms, but that the size of individual bristleworms isn't directly correlated to overfeeding. The mere presence of consumable matter is all that is required.