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That’s interesting, new practice to prevent the spread of Covid-19.For the benefit of Americans the reason I asked this question is because for at least some of my diving I,ands the other divers I am with are now required to carry a pony bottle instead of relying on another divers octopus. This is to prevent spread of covid - 19. I do not know how widespread this practice is yet but the day after I ordered my pony bottle the supplier phoned to apologize for being out of stock due to a lot of divers buying them. I have only used a (hired) pony a few times before and on those occasions it was slung on my d rings, where I could see the button pressure gauge and operate the valve.
I would agree 30 or 40cf. I personally suggest the Al 40. Same price as the 30, better characteristics in the water both full or empty. No noticeable weight difference in the water. Trims/streamlines better, I have never heard anyone complain about bringing too much air. Oh and btw, if there comes a time when you truly need a pony..... (not a shallow recreational look at the fishies dive. That’s what cesa is for) a bigger air supply is always better. A panicked diver can breath 5x more then their normal SAC and RMV.That’s interesting, new practice to prevent the spread of Covid-19.
In that case, and since you’re diving in cold water, a 30 or 40cu ft tank is what I’d chose. Big enough to easily ascend from 100 ft without worry.