I am not familiar with the Interpet brand of methylene blue or the appropriate concentration of that particular medication to use for your purposes.
However, the exact dosages are not that critical when using methylene blue for prolonged immersion so there is considerable leeway. You can "guesstimate" the dosage in most instances by the shade of blue that results in the treatment tank. You’ll want to achieve a nice royal blue tint to the water, but the shade of blue should not be so deep or dark that it interferes with your ability to observe the seahorses that are undergoing treatment clearly, or that it prevents the juveniles from clearly targeting and efficiently feeding on the Artemia nauplii.
If your juvenile seahorses are experiencing respiratory distress due to exposure to ammonia or nitrate levels resulting in methhemoglobin in their bloodstream, the methylene blue therapy should provide them with quick, effective relief.
Best of luck resolving this issue, sir!