I would use the aquarium salt. The Magnesium sulphate is used for dropsy, or perhaps as a GH
boost in a planted tank .... it is not going to do much for bacterial infection.
It depends on what goal you are aiming for as to the salt level used. For simple "stress relief" or for nitrite poisoning, a level of 0.05% is used.
<0.05% = 0.05 grams per 100 ml (0.1 liter). A 120 l tank therefore will need 60 grams. 1 teaspoon is ~5 grams.>
If you are treating diseases, you will need to go up to 0.3%. You need to get to this level slowly, increasing no more than 0.1% every 12-24 hours. 0.3% is good for most external parasites, and also some bacteria such as aeromonas. To treat finrot, usu. 0.1% is enough.
I sometimes use 0.5 to 0.9% for short period (15 minutes max) to stripe off the slime coat in heavy parasite infection, or for sedation when doing procedures to the fish. <A salt dip/bath.>
Before you contemplate salt or meds, however, you should check your water parameters. Fish usually get sick when the water quality had deteriorated. A largish water change will never hurt.
Also, white stringy poop & lethargy often indicates an intestinal infection. This is not well treated with meds in the water (or salt for that matter). The best treatment is antibiotic containing food. If you are in London UK, you will not be able to get antibiotics without a vet prescription.You can try the OTC stuff if you like, but clean water & salt is prob more effective.
More reading - although this talked about pond fish, it applies to goldfish as well: