I distribute fresh fish for a CSF (community-supported fishery), which means I have the option of taking home unclaimed shares. Last week it was 6 large dabs (similar to flounder). My piscine evisceration ability is rather crude. I made stock from the heads, fins, and tails. Perhaps I failed to remove every bit of unidentified organs, or to rinse “suffishently”. 
For whatever reason, the stock has a slightly bitter, mineral aftertaste. I have on hand both Better than Bouillon fish base, and a jar of crab paste. The latter was a recent impulse buy, as yet unopened. I have never used crab paste before. Which would be the better choice as first aid for the stock? My ultimate goal is to make gefilte fish, a first for me. I liked the ones I tasted as a child, which were made by two different home cooks. I want to make it as an homage to the late Dr. Oliver Sacks, for whom I had the highest regard. Gefilte fish was the topic of his final New Yorker essay.
fish stock is possibly the most tricky ‘stock’ to produce.
there is no save; toss it.
in my non-professional experience,
flat fish
oily fish - bluefish for example
any fish with a distinctive taste profile…
is not a good candidate.
any of the internals or gills is another thing than can muck up the works.
it sounds boring, and it is, but stuff like cod, haddock, hake… not exotic but reliably produce a fishy-tasting mild fish stock.
Hmm. It sounds like there is an unpleasant flavor. I don’t have that exact situation, but I have added a certain ingredient a bit too much and created an aftertaste, and there wasn’t anything I could do to “Take out” that flavor. So, instead, I overwhelmed the flavor with salt and the aftertaste just sort of disappeared.
I would partition your stock into 2-5 separate little bowl, and try to overwhelm your bitter mineral aftertaste with different ingredient. I would try a few things. Maybe: 1) salt, 2) ground pepper, 3) dried shrimp, 4) garlic or garlic powder…etc.
I’m going to guess . You did not remove the gills in the head . Did you leave them in ?
Very good educated guess. Anyway, I do love fish soup (soup based on fish). It can be very lovely.
Yes - I forgot to remove the gills. Am I sunk?
There might be some expert opinions for you . I don’t have anything except maybe add some bottled clam juice to balance it . Not sure if that is going to work though . Now you know whenever you use fish heads for stock always remove the gills .
I decided to chuck it. Even if I’d been able to improve the flavor, the color was ghastly. Lesson learned!
