Guppies with parasites seem incurable

[Sassyhippo]

Hello,

For several months I’ve been battling some sort of parasite. Guppies get skinny and die one after another. Included is 1 neon tetra and 1 purple rasbora. They’re both really skinny and can’t gain weight. Some fish have become skinny and lethargic but bounced back. Many have died. 
Things I’ve tried: general cure (2x), prazipro (2x), levamisole (3rd round), medicated food with general cure and epsom salt to promote passing the parasites. Multiple cleanings a day with Turkey baster removing poop, daily water changes around medication schedules. Aquarium salt almost constantly.  Now keeping water clean and watching after last levamisole treatment. Meds were spaced out a week or so each. 
Feeding: bbs, bloodworms, frozen brine, 2 different flakes, live black worms when available, some sort of frozen medley for tropical fish. 
tank parameters :

ph: 8.0-8.2

temp: 78°

ammonia, nitrites: 0ppm

nitrates: <40ppm always

Gh, kh: 4°, 6°

no decor, bare bottom tank. Sponge filter. I think the levamisole did some good as all but 3 fish are plumping up. Those 3 were extremely skinny though. 
 

Any other suggestions? Should I separate the skinny ones?

[Colu]

Internal parasites put a heavy burden on small fish some can die because of internal damage all ready done be the parasites I would feed them 3 time a day to put some weight on them and monitor the skinny fish for now 

[Sassyhippo]

8 minutes ago, Colu said:

Internal parasites put a heavy burden on small fish some can die because of internal damage all ready done be the parasites I would feed them 3 time a day to put some weight on them and monitor the skinny fish for now 

Ok will do. I was feeding twice but I’ll increase it. I think the rasbora might be barely able to see and struggles to catch food, but everyone’s eating. Nobody seems to be avoiding food on purpose.