yea they could have come in with the plants, the best thing to treat any parasite like that is salt. make sure its aquarium salt, and it needs to be at 0.3% thats 3 tea spoons per gallon, you need to start at 0.1% then increase to 0.2 in 12 hours, then after another 12 hours make it 0.3%.. i hope im making sense..
for example, if youv got a ten gallon tank, put in 10 tea spoons of salt.. then 12 hours later another 10 then another 12 hours another 10.
make sure you dissolve the salt in tank water before you pour it in the tank.