Yes, thaw them out before you put them in the tank. Some people will use a fork to skewer and weight the veggies down. Another option is to boil the veggies so they'll sink. And if you plecos are active at night, then put the veggies in at night. After a while, once they associate the veggies with food, you can start to add the veggies while the lights are on so you can see them feed.