I started thinking again about transformation spells (for overtly Disney reasons – curse you charming and distracting television) and it occurs to me that deprivation will affect a transformed target in an unsuitable environment very severely. It only takes 30 seconds to force a deprivation check, which escalates quickly in Ars Magica (see Ars Magica p.180). So this allows Muto to be used to inadvertently fatigue or kill, or perhaps the smaller fish form to be easily disposed of.
Transformation of the Vicious Beast to the Floundering Fish
Muto Animal 25, R: Voice, D: Sun, T: Individual
The targeted animal is turned into a small fish, size -8. Without a suitable habitat the fish will suffocate due to deprivation. Fish bones may be held by the target and dropped to end the spell prematurely.
(Base 5, +2 Voice, +2 Sun)
I suppose a similar spell could make a fish into a land animal, but that is such a niche concern. Then the same effect designed to affect a human target, with Voice range so it may be used on friends and foes alike.
Transformation of the Haughty Man to the Floundering Fish
Muto Corpus / Animal 40, R: Voice, D: Sun, T: Individual
The targeted person is turned into a very small fish, size -8. Without a suitable habitat the fish will suffocate due to deprivation. Fish bones may be held by the target and dropped to end the spell prematurely.
(Base 20, +2 Voice, +2 Sun)
If I were choosing this spell for a player character is likely not have the fish form so small. A shark gives better combative survival or a dolphin can also breathe normally and swim to reasonable depths.