Clarkii clownfish with white string poop

I need help on my clarkii clownfish. I had the clownfish for roughly 2 weeks and I noticed couple days ago it has white string poop. I was worry about it and so I called my lfs in which I bought the fish from. They told me that the fish is adjusting to the new fish food and my new specturm fish food is bad. I thought this guy from my lfs is very cocky and stupid because I know new life spectrum food pellet are the best food available for marine fish. Then I went to other lfs (little bit farther than my house), they told me that my fish got internal parasite and should be treated right away and Prazipro is not reef safe and could kill the coral that I have in my tank. The new lfs, they recommended me Dr G's marine aquaculture dewormer Rx. So far I bought this Dr G's marine aquaculture dewormer Rx. I'm not sure if my fish is actually eating it. How do I know if it eats the medicated food? I tested the water quality and everything came out good. Ammonia, Nitrite, and Nitrate are 0. My salinity is between 1.025 and 1.030. My phosphate is 0.5ppm. The new lfs, they recommended me to buy reactor or chemispure elite to lower my phosphate to 0 which I did bought it. So my questions are What is this white stringing poop in clownfish? Is it parasite? If so, what should I do? Or is it bad food? Is it stress? I don't think it is stress because it's the only fish in the tank. I don't have the money to create quarantine tank. I already bought the Dr G's medicated food. It has been two days since i bought it. I'm not sure if it eat the medicated food. How do i know if the fish is actually eating it? Because I watched the fish, it never took a bite of it. I thought it shy and so i left the room. Please Help.... I don't want my clownfish to died. We have spent alot of money on the tank already and my parents have zero knowledge in SW. I want to buy QT but my parents said, "You don't need it and look at my friend, he never use QT. Don't waste anymore money. Appreciate on what you have right now."