You will need to purchase food grade or high chemical purity calcium carbonate as shown below in the amazon link below.
In a container add 250ml of RODI water and 115ml of the powder to the water. Mix these thoroughly together. You will notice they will separate so will need to be well shaken prior to dosing.
To treat, you will need to entice the Vermetids to extend their mucus nets. I have found the simplest way to do this as Mike suggested is to use a turkey baster on the rocks, dislodging settled detritus while the flow is turned off. As soon as they detect the ‘food’ in the water they will extend their mucus. Be ready to add the calcium carbonate mix and turn the flow back on to distribute the ‘snow’.
Add 1ml per 100L (25 gallons)
You will notice as you watch that the mucus nets become increasingly white as the chalk adheres to them. Yes, all they catch is chalk. I’ve watched this happen and cannot decide if they die because they draw the chalk in and it clogs up their tube, or the mucus net gets too heavy to retract and they starve because they have not drawn any food in.
You cannot eradicate them after a single treatment. To control these things your have to be dedicated, and as Mike suggested, treat the tank every day for a month. This will choke up the majority of the tubes, but never rid of them in your tank completely. You may need to do the same in 3 months, or 6 months, again.