20 worms sounds reasonable. I just helped a friend move his SW
tank....and discovered his tank was infested with bristleworms. He only had a 20 gallon tank....and we took out at least 200-300 bristleworms from his tank and live rock. Yours most likely came in your live rock...living deep within...and mostly only come out at night.
It was a used tanked he bought...and I suspect overfeeding was one of the problems, which may have helped contribute to the bristleworm infestion. Nitrates being off the chart and a maroon clown which was double his normal size for a 1 year old clown were also clues.
In small numbers I have heard that bristleworms are beneficial to the tank.
90 Gallon Fresh Water Planted w/ CO2
7 Brilliant Rasbora, 6 Harlequin Rasboras, 3 Dwarf Neon Rainbows, 1 Red Tailed Shark, 2 American Flag Fish, 4 Skunk Botia Loaches
40 Gallon Breeder Salt Water: 4 inch DSB
(2/3 Agaramax & 1/3 Sea Flor), Bak Pak 2R Skimmer w/ BBX, 2 x JBJ PHs, Coralife Lunar Aqualight (2x96w), 30lb Base Rock and 40lbs of Live Rock.
Inhabitants: Purple Firefish, Pygmy Angel, Lawmower Blenny, 2 Cleaner Shrimp, 1 Peppermint Shrimp, Pistol Shrimp(HH), 1 Emerald Crabs, 3 Porcelin Crabs(HH), varying amounts and varieties of hermit and snails.
3 Gallon Eclipse:
1 Goldfish (won free at a fair...cost $50 in equipment)