Thanks for the quick reply, and that does work, but shouldn't there be a more concise way to do it?
This is what I have.
Predicate<ItemStack> predicate3 = new PotionHelper.ItemPredicateInstance(ModItems.itemName);
PotionHelper.registerPotionTypeConversion(PotionTypes.NIGHT_VISION, predicate3, PotionTypes.INVISIBILITY);
PotionHelper.registerPotionTypeConversion(PotionTypes.LEAPING, predicate3, PotionTypes.SLOWNESS);
PotionHelper.registerPotionTypeConversion(PotionTypes.LONG_LEAPING, predicate3, PotionTypes.LONG_SLOWNESS);
PotionHelper.registerPotionTypeConversion(PotionTypes.SWIFTNESS, predicate3, PotionTypes.SLOWNESS);
PotionHelper.registerPotionTypeConversion(PotionTypes.LONG_SWIFTNESS, predicate3, PotionTypes.LONG_SLOWNESS);
PotionHelper.registerPotionTypeConversion(PotionTypes.HEALING, predicate3, PotionTypes.HARMING);
PotionHelper.registerPotionTypeConversion(PotionTypes.STRONG_HEALING, predicate3, PotionTypes.STRONG_HARMING);
PotionHelper.registerPotionTypeConversion(PotionTypes.POISON, predicate3, PotionTypes.HARMING);
PotionHelper.registerPotionTypeConversion(PotionTypes.LONG_POISON, predicate3, PotionTypes.HARMING);
PotionHelper.registerPotionTypeConversion(PotionTypes.STRONG_POISON, predicate3, PotionTypes.STRONG_HARMING);
PotionHelper.registerPotionTypeConversion(PotionTypes.WATER, predicate3, PotionTypes.WEAKNESS);
That's a whole lot more code than a small line that gives all the same properties of an item. You know?