Hey, was thinking of feeding my fish cod liver oil. I know paul b feeds fish oils, but cod liver oil has a lot of vitamin a and d. Is this a problem if i soak the food in it and feed it once a day? Will the fish get liver problems?
Cod liver oil
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A, D, E and K vitamins are liposoluble and accumulate in tissues, which can lead to toxicity. They can be administered for periods, but not daily.
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Vitamins act as co-enzymes in metabolic processes, they "facilitate" the process but are not consumed by it, so the need for food is minimal. Once a week may be enough.
OP
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I thought about it. Im thinking this is better and cheaper. The fish are really crazy for the flake soaked in this. Never seen them get so happy for flake lol.
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I really have no idea if it will be useful or not, but I doubt it will hurt anything to experiment. 
Paul B
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No, it won't. It may give you liver problems but not your fish. Fish eat whole fish every day and a fishes liver can be 15 or 20% of the fishes weight. The liver is mostly oil so a fishes diet consists of about 10% pure fish oil. (give or take for the species)
If a 100lb shark eats a 20lb grouper, it is getting about 4lbs of oil. That is a lot of oil and fish should have a good part of their diet as oil. You can not overdose a fish with oil as you can with us.
Fish should not eat solid fats such as beef or chicken because they are cold blooded and those fats (from warm blooded animals) will not be able to circulate in their system and they will expel it. Fish have no solid fat, only oil which is why they are good for us to eat.
When I used to keep predators like morays, groupers, lionfish, etc, I used to inject cod liver oil into goldfish and shrimp to feed them.
By the way, sharks use the buoyancy of the oil in their liver to stay afloat since they have no swim bladder. Their liver is huge.
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The same stuff I take, whatever is on sale as the fish are not particular. I stick a pin in a capsule and put it on something dry like a freeze dried something. Never put any kind of oil on wet food as it does not penetrate because oil and water will not mix and oil floats on water so the oil will just instantly fall right off the wet food and float with absolutely none of it going into the fish. I know many people put oil on wet or frozen foods but between you and me, they are totally wasting their money. You can put a little on dry pellets and let it soak in for an hour. If you put that in your tank before it soaks in, you will have an oil slick and your skimmer will stop for hours. Ihave been feeding oil and taking it myself for decades, and look what I look like. 
With most things in this hobby a little common sense prevails. Just look at what fish eat in the sea. Most of us are not like fish even though some of us look like them. We don't eat whole fish with the guts, liver, bones etc. Fish do. They are cold blooded, that means not only they are cold, but have totally different needs and taste in clothes. So don't you take too much fish oil and never put oil on wet or frozen foods. Keep that a secret or thousands of people will learn they wasted a lot of money for nothing.
With most things in this hobby a little common sense prevails. Just look at what fish eat in the sea. Most of us are not like fish even though some of us look like them. We don't eat whole fish with the guts, liver, bones etc. Fish do. They are cold blooded, that means not only they are cold, but have totally different needs and taste in clothes. So don't you take too much fish oil and never put oil on wet or frozen foods. Keep that a secret or thousands of people will learn they wasted a lot of money for nothing.
Even in fish (and distinguishing, one thing are fatty acids, oils in general, and something else are vitamins), fat-soluble vitamins (A, D, E and K), when offered routinely and above their requirements, accumulate in their fatty tissues (and liver) and may lead to toxicity.
About vitamins in fish diet:
Dietary vitamin requirements of fish and shrimp - Food and Agriculture
Highlights:
"5.5.2 Vitamin toxicity
In contrast to water-soluble vitamins, fish and shrimp accumulate fat-soluble vitamins under conditions where dietary intake exceeds metabolic demand. Under certain circumstances, the accumulation is such that a toxic condition (hypervitaminosis) can be produced. Although such a condition is not likely under practical growing conditions, hypervitaminosis has been experimentally induced in fish. Signs of toxicity reported include:
VITAMIN A
Salmonidae - Reduced growth and hematocrit, severe necrosis / erosion of the anal, caudal, pelvic and pectoral fins, scoliosis, lordosis, increased mortality, pale yellow livers.
D VITAMIN
Salmonidae - Reduced growth, lethargy, dark coloring
Channel Catfish - Reduced growth, poor feed efficiency
VITAMIN E
General fish - reduced growth, toxic hepatic reaction, mortality"
It is obvious that such assessments refer to the conditions of commercially grown fish and crustaceans, bred in captivity, which do not represent conditions in the aquarium or in the natural environment, however, as it is our responsibility to feed them , it is best not to abuse, even with good intentions.
Best Regards
About vitamins in fish diet:
Dietary vitamin requirements of fish and shrimp - Food and Agriculture
Highlights:
"5.5.2 Vitamin toxicity
In contrast to water-soluble vitamins, fish and shrimp accumulate fat-soluble vitamins under conditions where dietary intake exceeds metabolic demand. Under certain circumstances, the accumulation is such that a toxic condition (hypervitaminosis) can be produced. Although such a condition is not likely under practical growing conditions, hypervitaminosis has been experimentally induced in fish. Signs of toxicity reported include:
VITAMIN A
Salmonidae - Reduced growth and hematocrit, severe necrosis / erosion of the anal, caudal, pelvic and pectoral fins, scoliosis, lordosis, increased mortality, pale yellow livers.
D VITAMIN
Salmonidae - Reduced growth, lethargy, dark coloring
Channel Catfish - Reduced growth, poor feed efficiency
VITAMIN E
General fish - reduced growth, toxic hepatic reaction, mortality"
It is obvious that such assessments refer to the conditions of commercially grown fish and crustaceans, bred in captivity, which do not represent conditions in the aquarium or in the natural environment, however, as it is our responsibility to feed them , it is best not to abuse, even with good intentions.
Best Regards
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