I've never seen aquifer turtles, and the wiki claims they don't exist.
Really? If by "seen" you mean the display showing a turtle in the aquifer pond tile, I'd have to agree (but ISTR reading somewhere [yeah, I hate that I can't report where] that such displayed fish are unrelated to what a fisherdwarf might catch). Catching pond turtles from a hole channeled into an aquifer is SOP for my forts, though.
I routinely set zone-only fishing and punch an underground aquifer hole below for a secure source of water and turtles (habit formed in my early days, when werebeast infections were more *fun* than I wanted). I've watched dorfs fish at the aquifer pool and then haul a turtle away to the fishery or raw fish stockpile, with or without a covering grate or well.
Sometimes the aquifer pool seems to produce forever, sometimes they seem to play out after a while, sometimes they come back. Sometimes, zone designation reports zero fishable tiles from day one, even placed below a surface stream or pool; on at least one such occasion I couldn't catch anything from the surface stream, either, and had no shells until a giant snail wandered into a trap. (knowing no compelling reason to update, I play 43.05)
The wiki I read clearly states "Pond turtles can also be found by digging into an aquifer"
(
https://dwarffortresswiki.org/index.php/DF2014:Pond_turtle)
I suspect the OP lack of "fish" is either unfortunate placement with respect to the invisible 16x16 "fishable grid" or just bad luck. Sometimes I can't find a fishable tile anywhere, even in pre-existing surface waters.