Help! Red flatworms

[Redstratplayer]

My first saltwater tank a 20g long now has flatworms

I think this is from water the cardinals and Priolepis were baged in at my LFS.(As soon as i can i am going to go rip them a new @%# hole)

Ive had this tank set up for 4 months.
20lbs live rock
fish
1 green clown gobie
2 bangi cardinals
1 Priolepis semidoliata

coral
1 dendro
1 acan
1 porcillapora
6 polyps BPE (brown people eaters)

I love this tank and now it has flat worms.
Whats the first step?