A Goldfish Simulacrm needs a Fishbowl
Actually, I'd like to share my perspective. My Wizard uses Similacrum's fairly extensively to extend his power base. They are less expensive and more trustworthy than people, and it's few and far between when a powerful person casts Trueseeing or some such on one of my simulacrums who Amatar introduced to a new town and carefully assisted its rise in prestige and authority (though if it happens, it might make for an interesting game (there are many key individuals who are similacrums...heh. He rarely uses similacrum as a weapon). He carries similacrum's he uses as tools (like his rust monster sim) in his Mirror of Life Trapping which he keeps in his portable hole. If he dropped it in the Hole, it would suffocate.
First...depending on what your similarcum is, you might have trouble getting it inside. At the largest size, a Bag of Holding can carry 250 cubic feet, which is approximately a cube 6.3 feet on each side. You could fit a medium humanoid (which is less than 2 cubic feet total) in there but not much room for a larger creature I'd say.
Second. Your simulacrum would die of suffocation. Just because we know it's not living doesn't mean Power Word Kill, Fireball, Chocking Cloud, poison, or...NO AIR won't kill it. It's not dead...it's a "simulation" of life. It needs to eat, breath and sleep unless the original it is "simulating" doesn't need to. The spell description doesn't mention the rather vast power of not breathing, eating, immunity to gas, poison, etc, so it would be safe to assume it doesn't. Similarly, it doesn't say in the spell description of Magic Missile that "after all missiles have struck, three buxom women in chainmail bikini's appear to massage the caster." We therefore assume that doesn't happen either.
Would a simalacrum of a goldfish die on your coffee table? I think so. Unless, that is, if the afformentioned coffee table was underwater...then you're really in trouble!!!!
If the simililacrums DIDN'T breath eat or sleep, my Wizard would be in a bit of trouble as people might start to notice they haven't eaten or slept and only breaths when they plan to speak. Might put him out of business.
Final note: The simulacrum spell is both more powerful and weaker than it was in AD&D. Its change was most likely an attempt to control enterprising characters like Amatar, but didn't really do so. Formerly the old spell started with a base and you would randomly roll to come up with a number which represented (clearly stated) a percentage of the targets level, abilities and knowlege (it's major usage was information gathering...you'd make a simulacrum of the badguy and obtain...say 65% of everything he knows!!!!). This element of knowlege is NOT mentioned in the new system. But it is vaguly hinted at. Basically the spell is weaker AND stronger. You dont' have to roll for a 50% chance to see if they have "this" power or "that" power. He's just at 50% straight across the board. My interpretation is to assume a similar 50% of the subject's knowlege base.
I will admit that since AD&D I have NOT made several simulacrums of the "darklord" to learn all his secrets...using them to give conflicting commands to his troops, then gather a party of adventurers to take him out now that we know most of his plans and secrets. But...I'm sure I could if I wanted to traumatize my DM.
Storme