Bispira worms are great at dusting out the solids from your water. I do not think that there is a limit to how many you can have as such, actually.
If you get more than one they can come to the conclusion that your tank is the perfect habitat to create baby-dusters in and then they might spread fast. I only keep & encourage a colony of the small tubeless white type of bispria because I can then keep Vayssierea Felis (type of tiny nudibranch), and together with peanut worms, they act as a great cleanup crew. If I could have peanut worms in every tank I would never even consider snails, shrimps or crabs as cleanup crew because I honestly like the peanut worms much better.
Great looking tank btw., What is the thing in the middle? It looks beautiful, is that a sarcopython? I am not that into coral names as I am generally obsessed with Macrodactylas (LTA's) and my clownfishes.