Any connections you have, whether it's a coupling, "T" or anything else, silicone caulk the snot out of it and let it set up. Those corregated connectors aren't known for being air tight.

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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).