This article from Star K says in part:
Similarly, fish gelatin must be produced from a kosher species of fish
if it is to be considered kosher. The use of fish gelatin with meat
foods poses an interesting question. As previously noted, the Shulchan
Aruch (Yoreh Deah 116) prohibits the cooking of meat and fish together
due to health concerns. We tend to be more stringent when dealing with
possible health issues than with concerns of Issur (prohibited
substances). Therefore, there is a question among the commentaries as
to whether or not the rule of one in sixty nullification applies to
unhealthy substances just as it does with prohibited substances. The
custom is that unhealthy substances become nullified at a ratio of 1
to 60 (see Nekudas Hakesef, Yoreh Deah 116 & Pische Teshuvah).
There are many reasons for leniency in the use of fish gelatin
together with meat.
Many rabbinic authorities are of the opinion that the nature of some
foods has changed, thus rendering the mixture of meat and fish no
longer unhealthy (see Magen Avrohom Orach Chaim 173:1, Teshuvos Chasam
Sofer vol:1 #101).
Furthermore, there is a rationalization that not all fish would be
considered a dangerous mixture with meat. It may be that only the type
mentioned in the Gemora (Binita) is unhealthy (see Pische Tshuvah,
Yoreh Deah 116:3). It may also be maintained that the unhealthy
aspects of fish cooked with meat are found in the flesh gelatin is
made). [I suggest the author meant to write, “It may also be
maintained that the unhealthy aspects of fish cooked with meat are
found in the flesh while gelatin is made from the skin and bones.”]
Since gelatin may not have fish flavor, it may not harbor the harmful
effects that fish may carry (see Pische Tshuva, Tshuvos Sride Eish
vol:2 #67 re: cooking beef in fish oil).
With this same reasoning, we can say that gelatin can be batel
(nullified) with a majority of other food ingredients and can be eaten
with meat (according to R’ Aharon Kotler, zt”l regarding animal
gelatin and milk).
For these reasons, it may be acceptable to use products containing
fish gelatin with meat, or use the same reasoning to allow products
containing animal gelatin with fish.
Thus there is no practical point in labelling marshmallows as OU fish.