good bristleworm or bad fireworm?

I work at a pet store that carries saltwater fish, corals & live rock. Fire worms have been turning up in our tanks and tanks of our customers from live rock that came in from a certain supplier 3 years ago that we no longer deal with. The large ones are easy to identify and many people have seen fish killed and been stung themselves, thier fingers/hand becoming extreamly painful for hours or even days. However we are now finding small red worms with white bristles in large numbers in empty tanks that have never had rocks in them (only arragalive), but are on the same system as tanks with fish/invertebrates/live rock. Are these small fire worms, or harmless bristleworms? We don't want to remove them if they're not harmful to our stock.

the worm in the photos measured 4.75 inches. most are 1" - 3"

we've deffinatly encountered those worms, but these smaller ones are solid red or have dark lines down thier dorsal (food in the gut?) and are much skinnier. They are mostly found in the arragalive or inside empty snail shells but occasionally in rock as well. Really wish we had a better camera.