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>TotT
I feel like a similar question was asked a thread or two ago, but I don't know for sure.
I've had the idea that the pokeball would generate some faux-water field to suspend/sustain the fish pokemon for use on land (said field would have a limited range, a typical battlefield's worth.
I highly doubt that's the case, as I don't recall anything like that showing up in any official media, but that's how I'd do it.