On p 77, the rules about Creo says: "Conversely, magically created food only nourishes for as long as the duration lasts, and someone who has eaten it becomes extremely hungry when the duration expires"
My interpretation of that is that an magus can not "cheat" true hunger by creating apples (without vis) with sun, or even moon duration. Therefore, it seems only logical that you can not have true nourishment from air either.
That makes the spell Lungs of the Fish, (p122) somewhat… fishy. The effect "Turns water into air as it enters your lungs, allowing you to breathe water as you do air.". But the water isn't truly air. And therefore the targets lungs should not be able to take nourishment from it. You would drown with water in your lungs when the spell ends. Likewise you shouldn't be able to make an CrAu-bubble to be able to breathe.
A MuCo AnAq requisite to get gills should on the other hand be possible.
For one thing, Lungs of the Fish is Muto, not Creo. Different limits. Also, all that air probably does turn back into water at the end, but luckily for the caster medieval understanding of breathing wasn't exactly great and you've already exhaled all that water so there's no water-air left in your lungs unless the spell ends at a bad time (in which case you were dead anyway).
Lungs of the Fish also has some interesting notions for what the target of the effect is, and how that combines with it's duration.
There is an argument to say that the water that is affected by touching the magus is transformed, and will stay transformed for the duration, but the next breath is not. So the magus transforms the first breath, but needs to keep casting it to transform new water as it enters the lungs. LotF isn't the only spell however which has this slightly different view on R,D,T guideline interaction either (the others escape me).
Getting back to your question - the Muto effect means that the breather will take sustenance from the altered water. I've liked the idea that you can take sustenance from a magically created substance as long as the original material has "left" the body within the duration. A day is highly suspect for food, but fine for air. A month for food is fine.
I believe trying to fully explain the Creo limits is a bit like opening a can of worms.
You see, if a Creo food of a month duration would not trigger hunger at the end of the spell, it should be possible by the same token to challenge non-permanent healing spells. Why is natural healing suspended when under a non-permanent healing spell if a moon duration created food can provide sustenance ? After all, the temporary created food was able to be absorbed and sustained a human. Under a healing spell, the cells are getting replaced over the duration of the spell, so why should the wound reopen - I know the Medieval paradigm does not acknowledge cells and cells replacement, but I am guessing that the understanding of the digestion process is equally misunderstood.
I prefer to keep the rule simple: temporary Creo cannot provide permanent sustenance or healing. It might be arbitrary, but it is simple and does not lead to lengthy argument.
But Lindenius has a point, Muto is temporary, so the "sustenance" provided by air can be questionned as well. The only explanation I could venture is the paradigm on breathing/air concept does not know how the air is absorbed by the body (and transformed by it), so there is no issue regarding the air "sustenance". But I would rather let other experts give a more definite answer.
Well... Healing has always been an exception in its own right. I mean, eating magically created food doesn't "suspend the digestion process" in the way healing spells suspend the healing process. Basically, non-Ritual healing actually does, as you put it, prevent cell regrowth in the area. Still, I don't know all the paradigm-y stuff perfectly, and I'd be fine with either SG ruling, whether it's "the month duration food has already gone through your body completely so all that disappears is feces" based on the precedent of Hermetic interaction with Fatigue, or "all that food has provided energy, so when the duration runs out you lose all that energy value, resulting in massive Fatigue if you've eaten recently and possibly death if you haven't" based on the precedent set by magically created animals eating mundane food.
I guess it's all up for interpretation. So the OP is right, the spell is fishy, but its existence is justifiable within the rules.
It's a legacy spell, not without some problems. I have less of a problem of this spell being used to allow someone to breathe underwater for a while than I do someone creating Moon duration mass quantities of food. If you accept that moon duration food doesn't leave you starving after the effect should have expired, you're basically creating a situation that allows covenants to manufacture wealth for free. The point being is that the side effect of allowing Lungs of the Fish to operate as written compared to say making food are asymmetric in the impact they have on the saga and the overall cosmology (for lack of a better word).
Well, this isn't true, because there are Creo Corpus spells that allow the body a bonus for healing. If these aren't healing spells, I'm not sure what they are. The only canonical spell that "heals" or gives the appearance of healing without Vis is Bind Wounds. I suppose someone could invent Sun duration versions of spells that heal Light, Minor, Heavy Wounds, but such spells are unlikely, and so the only spell where the restriction of healing spells suspending the healing process is effectively Bind Wounds.
Unlikely does not mean it's any less possible than any other formulaic spell within Hermetic guidelines. The example was to illustrate what Hermetic magic can do, not what a given magus might actually want to do. And yes, you can give somebody a bonus to their entirely natural healing process, which makes sense, as Creo magic is not Faerie wizardry and the effects things have had on the world last even when the effect expires. Using Creo to make a horse the swiftest a horse can be for a limited duration does not cause it to move backwards to where it normally would have been once the spell ends, and using Creo to create fire leaves burns on the foe that don't disappear at the end of the spell. In much the same way, those Moon-duration spells that improve your body's ability to heal naturally during the duration will stop improving the body's natural healing capacity after the spell ends, but that won't undo all the healing your body did while its healing ability was magically improved. A good way to look at insta-heal spells done non-Ritually is that they fill in the space with flesh and artery connections and whatever in the same way your body would, so not only does your body not bother healing itself, it can't, because all that magically placed stuff can't just be regrown through. So when it disappears at the end of the spell, the wound basically re-opens, with no work done by the body to heal itself.
Do you have a different interpretation of how healing spells and Recovery-boosting spells respectively work? I'd like to hear it.
Fish can 'breath' air, therefore it is breathable. People cannot breath air, therefore air is obviously the form of the breathable material that people can breath, while water, another element, is not. Magical can transform one element into another more efficiently than it can items which are mixtures of the 4 elements, and as a result, obviously can make the water turn into air for a time. It's probably a spell which should technically be a rego if they ever REALLY through about it, but all it's doing is transposing the 2 elemental planes (Water and Air) for the magus.
It is probably possible, using this as the base idea, to make it so magi can breath Fire (fire elementals can) and Earth (Worms can, ants can), though they'd subsiquently suffer the side effects of those elements (getting wet for water, burning to death for fire, being unable to move for earth)