Vitamins in teleost freshwater fishes are another way to spend money. But before you buy vitamins for freshwater fish, please put the money in an envelop and mail it to the Department of Labor. It will do you about as much good there as it will in your fish tank.
Freshwater fish are inundated by water through their skin and gill. Indeed, if they never took a single sip of water, they would always be over-hydrated. And so they are. Their kidney is engineered to excrete colossal amounts of water all the time, while recovering precious electrolytes and solutes. The freshwater fish never does take a sip.
So how do the vitamins get into the fish?
“Indeed, they are absorbed!” reply the marketing weasels.
“But nay” say I, “The fat soluble vitamins cannot passively cross the gill membrane nor the skin.”
“So perhaps the water soluble ones can!” Exclaim the marketing weasels.
“But they don’t!” I say, “Because as soon as an organic water soluble vitamin encounters another organic molecule of almost any type it becomes bound out of solution, not to mention that most of these vitamins are already so unstable they must be kept in brown glass bottles. How long do they actually last in the water, so that they might be absorbed by the fish?” I query.
“Less than five minutes.” Comes the sheepish reply.
“Perhaps we could recommend that the vitamin-fortified liquid we have put so much ‘R&D’ into could be applied to dry food and fed.” Suggested the other weasel, nodding hopefully.
And so they did.