I noticed that when looking up other sturgeon pictures in PD, but the sturgeon shown in the photo certainly isn't a sterlet, as they're flatter and with a longer, thinner snout
@Mr Gharial@Kakapo I am pretty sure there are one or more sterlets in the pond, but this was a substantially bigger sturgeon with a different head.
I also asked in an identification group on FB and they seem to think either Russian sturgeon (Acipenser gueldenstaedtii) or Japanese sturgeon (Acipenser schrenckii).
Unfortunately I would probably need much clearer pictures to have certainty, but the pond doesn't make that easy.