Help. Parasites? Stress cause this?

[Mandalynn87]

Hello. I've had my f8 puffer about 2 months. He/she's in freshwater water, but is about to start slowly moving to brackish.
Ive never seen signs of parasites before. Hes always had normal poo. But last night I switched out his substrate for caribsea super naturals superscapes sand. I went slow. But got upset because he was obviously stressed . I didnt even put the plants back in. They are sitting/floating. I was worried about him.

This morning I see a string out his butt. The top seems white or clear. Bottom is poop color. I over worry but im sure this is a parasite. Its just strange its immediatly when I changed the substrate. I did notice him pooping when I was finishing up his substrate. And it was normal last night and every day before. He is a happy eater. He sees my tongs and goes. He eats bladder snails a few times a week, my colonies is much bigger thanks to a pick up yessterday. And also gets Krill thats sectioned into pieces and bloodworms . He gets blood worms the least option. Once or twice a week. And usually gets the snails with something else.

Also because it was clear he was stressed I left a small amount of Pebbles under some areas of the new sand. It was a flat layer and not enough to cover the entire bottom.. I see a patch ill need to get some out. But mostly gone

Nitrates are around 5.nitrites are 0 Ammonia is 0. Hard water. Ph about 7.7 to 8. I cant tell the KH. Color is hard to read. Either ideal or high. . I use the master test api for ammonia and lately the terta strips for the rest. With occasional api master test for everything ( I'm nervious about the strips but its alot with 3 tanks)

IN THE TIME I WROTE THIS IT IS NO LONGER ON HIS BUTT but I did get photos.

I DID READ STRESS CAN MESS WITH DIGESTION AND CAUSE MUCUS TO CAUSE POOP LIKE THIS BUT IM SO OUT OF MY FIELD HERE. LOL AND I FELT LIKE HE WAS STRESS POOPING WHILE I DID HIS SAND. WHICH IS WHY I DIDNT FINISH PUTTING THE PLANS AND STUFF IN. But I did get him from petco ( not my usual spot but i instantly fell in love when i saw him, and i had a cycked tank at home, soooo) . I'm told they treat all tanks with melafix, prina fix and ich, but i know they need to invest food.

Also. I fed him a big Krill meal before bed. And he had 2 dnails befire i started scaping. Seeing him eat made me feel like I didnt kill him, though it's silly I know. So his belly is still full as that was less then 7 hours ago.

[Pufferpunk]

He looks extremely healthy to me, I wouldn't worry about 1 white poop.