we did a quarter water change yesterday and today the thing doesn't seem to be as big, it looked like it was falling off. Does this happen with parasites?
also what will happen if we don't do anything else to treat it? Will the parasite spread? I want to treat it with salt but don't want to kill the plants in the tank, or move the fish for a dip as she is pregnant and I don't want to stress her. Will a few more days hurt?
Depends which parasite it is. If its a parasite at all.
Just keep a close eye on the thing.
Look out for parasite signs such as flicking and rubbing, excess mucas on body or gills, darting or erratic swimming, laboured breathing or gasping, red pin prick marks or red sores on the body, pierce wounds.