Treating internal parasites

I recently got a betta and when I got him there's a white stringy poop hanging in his anus. I'm medicating him using Azoo's Anti Internal parasite, how should I know if the parasites are gone? He's still swimming jerkily and showing stress stripes

Does he still eat? Usually you will know if he is parasite free when he starts putting on weight and his poo is red/brown.
What's his tank like? Is his tank filtered, heated and cycled (Nitrogen cycle)? If the tank has not cycled and isn't heated that could be why his stress stripes are there.

the stress stripes are not always present, it seems to be appearing only when he starts swimming jerkily. He still eats a lot. there are no stringy poops now but he's always flashing. He's in a 2.5 gallon tank with sponge filter but no heater since I'm in a tropical country and temps here are pretty hot already. He's with two nerite snails, do you think the nerites are putting too much bio load in the tank? and maybe the flashing is caused by ammonia poisoning?

Possibly, a 2.5 gallon tank isn't all that big to begin with and most people will usually have only a colony of shrimp in there or some plants and no fish. How often do you change his water and how much each time?
If you can test your water and post the results that would go a long way towards solving the problem. Perhaps post a pic of your boy as well so we can see his body condition.

I don't really think ammonia burn would cause the fish to flash, but I'm not completely sure. If you can't visibly see anything wrong with him then try taking a flash light and shine it on him while his tank light is off. If you can see a gold dust-like sheen then the problem might be velvet.

For now, I would change about half his water every day or every other day, and maybe try adding some aquarium salt. Getting his temp up the about 78 should help as well. Usually heat, salt and water changes will cure most simple illnesses

his poop is stringy and white in color yesterday, I've transferred him into a qt tank (1.5 g) and treated him with azoo's anti internal parasite, his not flashing in there. I'll post a pic later. His body seems in good shape

Here's the pics. he even made a bubble nest in the qt. he's not doing that in his 2.5 gallon tank