Can a player wear armor while wearing the Fish Suit?

Question

In 5e Spelljammer: Adventures in Space - Astral Adventurer's Guide there is a magic item called the Fish Suit which does:

This bulky suit, which fully encases your head and body, takes 1 minute to don or doff. While worn, it enables you to breathe in an airless environment and renders you immune to the harmful effects of any gas that surrounds you. The suit also grants you a swimming speed equal to your walking speed while underwater, or a flying speed equal to your walking speed in an environment with no gravity.

One of my players has acquired one of these suits and wants to be able to wear it on top of their light armor. As a GM, should I allow this? And if so what about medium and/or heavy armor?

Answer 1

Fish Suit is not armor, so it can be worn with armor; the DM can overrule this if they don't think it makes sense

The DMG categorizes magic items (p. 133) as follows:

Types of magic items include armor, potions, scrolls, rings, rods, staffs, wands, weapons, and wondrous items.

The Fish Suit is categorized as a wondrous item in its description, not as an armor.

Fish suit
Wondrous item, Very Rare

The only limitation in the DMG is for wearing multiple items of the same kind (p. 141):

Use common sense to determine whether more than one of a given kind of magic item can be worn. A charac­ter can't normally wear more than one pair of footwear, one pair of gloves or gauntlets, one pair of bracers, one suit of armor, one item of headwear, and one cloak. You can make exceptions; a character might be able to wear a circlet under a helmet, for example, or to layer two cloaks.

Armor and Wondrous Item are different kinds of magic item, and there is no rule forbidding wearing different kinds together, so they can be worn together. While the fish suit is donned and doffed, like an armor, it is not one, and thus does not conflict with wearing one, rules as written.

Ruling on this

Because of the donning and doffing clause, which is normally reserved for armors, it feels as if this item would have been better designed as a magical armor; that however would have meant you could not wear it with other armor usefully, and would have a negative impact on the characters AC in environments where it is needed. Other features that allow breathing in space, such as the 2nd level air bubble spell that allows breathing in such environments for 24 hours have no such downside. Forcing the players to forgo their armor to wear it will weaken them compared to these. It's possible that this item was set up as a Wondrous Item intentionally so you can combine it with armor.

The rules-as-written conclusion still feels somewhat unsatisfying, because common sense tells us you should not be able to wear two bulky outfits at the same time.

If the DM does not want to conclude these items will magically adjust in size to accommodate this, like is assumed for magic plate armor that somehow fits both a stocky dwarf and a lithe elf, we can look at some real world equivalents to inform a ruling.

In the real world, you typically wear gambesons or some other padding under chain mail or plate armor. The game assumes, these are already included with heavy armor. But Fish Suit, which from the picture looks even larger and more bulky than heavy armor, is no armor. So maybe it does not include such undergarments. That would allow at least for wearing light armor together with it, maybe supple medium armor like chain mail, if you feel it should not fit over plate.

I think it would not be unreasonable for a DM to expand the "common sense" rule and decide that wearing it conflicts with wearing armor because of the bulk it takes up on the wearers body, especially with heavier types of armor.

Answer 2

The rules-as-written seems to allow it, but ask your DM anyway.

There are rules for stacking certain kinds of magic items on your body:

Use common sense to determine whether more than one of a given kind of magic item can be worn. A charac­ter can't normally wear more than one pair of footwear, one pair of gloves or gauntlets, one pair of bracers, one suit of armor, one item of headwear, and one cloak. You can make exceptions; a character might be able to wear a circlet under a helmet, for example, or to layer two cloaks.

Giving this a strict reading, we can see that Armor and Wondrous Item are different sorts of magic items. However, the real gestalt of this passage is to leave this sort of thing in the hands of the DM, and for good reason: the DM is responsible for rationalizing how the world fits together, even details like this. The first step on the basic loop of play is:

The DM describes the environment. The DM tells the players where their adventurers are and what’s around them, presenting the basic scope of options that present themselves (how many doors lead out of a room, what’s on a table, who’s in the tavern, and so on).

So the DM must be able to make sense of how your clothes fit together, or at the very least, they get to decide if they care. And given the illustration provided, I wouldn't be terribly surprised if the DM were to put some limits on this, particularly around some of the bulkier armor types see image below). While the rules-as-written typically serve as a baseline for expectations, this is the sort of thing that I would encourage you to just ask the DM ahead of time, rather than forcing them to make a ruling in the moment.