New to aquariums - clownfish with stringy white poo

I realize that there are tons of forum posts on this, but I can't find one that is exactly like my situation.



For a while, in my tank I've had

1 Chocolate Chip starfish

2 Clownfish

and 1 yellowtail damsel.

Last night I woke up to the damsel deceased, floating at the bottom.

Today, my clownfish have been vibrating near each other, and now the larger one had white stringy poo. I moved the larger into a small keeper, but it immediately caused them both stress so I put the larger one back.



I have a 10 gal aquarium and am going to my store today to do a water test and see what I could get from them. I heard Prazipro is good, because white feces means possible parasites. I'll also do a large water change when I get back, but is there anything else that I can do?



Thanks