How are you trying to get a siphon? I used to insert a air hose up and into the curve at top and then you juts suck the air out. It should stay and keep the siphon.
__________________ 180g- Mostly BIG fish and some coral. ~80g Nuvo- My coral tank with "happy fish"~ 90g- FOWLR Not the not happy type of fish~ 125g- Freshwater Malawi Cichlids ~10g- Nuvo- The refugees from the Ich of '18