For starters I would use the bigL as it is mild but will kill camalanus worms without harming anything else in the tank. They can be a major problem to fish and spread very easily.
If the fish still don't pick up in condition then I would do the Kusuri but be aware it may kill fancy shrimp and definitely will harm snails. Watch the tank more carefully if you have a major snail problem especially the trumpet snails as hundreds can exist in your gravel and that many dead bodies can create an ammonia spike.