IMO vitamin blocks are a gimmick, and pretty much worthless if one is attempting to supplement a fish with water soluble vitamins via the water column, especially in the case of Vitamin C which at the rate used in one of these blocks probably breaks down as fast as it enters the water column. While Vitamin C is an extremely important nutrient, especially for fish under any type of stress, the most efficient way for a fish to absorb it is via its diet.