I'd go 1 1/4". Use the longest lengths possible to minimize the number of connections underground. I would also put a stake where those connections are for future reference. I've had a 1" line pull apart due to heat/cold cycling. That had a hose clamp on it, but no glue.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).