I was eating a bowl of "Total" cereal, and my taste buds detected a piscean taste. Are fortified cereals obtaining their vitamins from animal sources?

Apparently, the added calcium in Total is in the form of calcium carbonate. The calcium is from limestone. It is not animal bones or chalk.

vegweb.com/index.php?topic=11784.0 webpage no longer exists

I have found that the B-12 complex is obtained from cow intestines.
http://freetheanimal.com/2010/09/dr-seale-any-b12-present-in-animal-foods-is-only-because-of-bacterial-contamination.html

Vegans consume no animal products so they must consume vitamin B12 fortified foods or supplements. There are several foods popular with children that are fortified with a vegetarian (NON-ANIMAL DERIVED) vitamin B12 such as breakfast cereals (Total, Nutri-Grain), Red Star Brand Vegetarian Support Formula nutritional yeast, meat analogs, and some soymilks. Most children’s multivitamins contain vitamin B12.

https://www.vegansociety.com/resources/nutrition-and-health/vitamins-minerals-and-nutrients/vitamin-b12-your-key-facts/what-every-vegan-should-know-about-vitamin-b12

Total contains "salt lactose," which sounds bovine to me.

"Ingredients
Whole grain wheat, sugar, calcium carbonate, corn syrup, salt lactose, distilled monoglycerides, vitamin C (sodium ascorbate), zinc and iron (mineral nutrients), vitamin E (tocopheryl acetate), niacinamide, calcium pantothenate, annatto extract color, vitamin B6 (pyridoxine hydrochloride), vitamin B2 (riboflavin), vitamin B1 (thiamin mononitrate), folic acid, vitamin A (Palmitate), vitamin B12, and vitamin D. Freshness is preserved by BHT, a synthetic antioxidant (preservative)."
https://en.wikipedia.org/wiki/Total_(cereal)

Not so! It's piscine....I think. This is so technical I'm not sure it's what's in Total, but does relate fish to salt lactose. I'm "swimming" in TOTALly unknown waters, however.

"Fishmeal extract bile salt lactose agar (FEBLA), a new differential medium for enteric bacteria was developed and evaluated for its ability to grow and differentiate lactose fermenters (LF) from non-lactose fermenters (NLF) in comparison with MacConkeys agar."
http://www.ncbi.nlm.nih.gov/pubmed/16924840

Total is not included in any of the three lists of vegan breakfast foods I could find so it is entirely possible that the fat soluble vitamins are animal derived.
However, just to introduce a new perspective on the same question, has this off flavor been verified by others in your residence ? If we were conducting a study, our observations have to be verifiable and repeatable.
If not, there are several reasons for dysgeusias or altered sense of taste, from effects of medications being taken to hormonal changes ( pregnancy, monthly cycles ), dysosmias ( altered sense of smell; both senses are closely tied together and aberrations in the sense of smell will significantly alter flavors ), thyroid disorders and many others.
From the list provided by qp and star_gazer, nothing really stands out as a probable source for this taste, unless we consider rancidity as a cause. Oils from any source, including those in natural grains undergo oxidation and eventually go rancid, no matter the presence of antioxidants. I am sure you did not open an expired box. Below is a link of a study measuring hexanal as a measure of rancidity in low fat foods such as oat cereals. It found that the addition of antioxidants reduced, but did not eliminate rancidity. Of course, taste perceptions vary greatly and perhaps some people will perceive rancidity as a fishy taste.
Another thing could be the natural sensitivity of some to certain tastes. For instance, some people perceive bitterness acutely while others do not. A link below mentions tasters, super-tasters and non-tasters in relation to bitter foods. Perhaps you are sensitive to some chemical tastes in the cereal not perceived by others.
A key question would be if this is something new and if off flavors are detected in other foods as well.

http://www.peta.org/accidentallyVegan/VeganShoppingQS-breakfast.asp?Category=Breakfast%20(Dry)

http://www.chezbettay.com/pages/breakfast1/brkfst_cereal1.html

http://www.emedicine.com/ent/byname/Disorders-of-Taste-and-Smell.htm

Vitamins may be synthethically created from coal derivatives or naturally isolated:
http://www.vitamins-nutrition.org/vitamins/natural-vitamins-synthetic.html

B-12 is generally created by bacteria, Vitamin D is nearly always from animal sources, and the rest are created synthetically.

If you are concerned about the possibility of the sugar you use having been treated by bone char decolourisation then you have several choices. If you want white sugar and sugar from sugar beets is available then use that. If you are not sure if the local sugar is from beets or cane, write to the manufacturer and ask them, adding that, if it is cane sugar, whether they still use bone char. If you want brown sugar, do not assume that it is inherently untreated. Many brown sugars are actually white sugar which has been coated with molasses. Again, check with the manufacturer if you are concerned.

http://www.sucrose.com/bonechar.html