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 Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.