25 mm Uniseal.
A Aquaseal, also known as a Uniseal is used to create a water tight seal around the circunference of a pipe going through a hole on a tank or aquarium.
Uniseal is also available in the following diameters:
Seals work great, one had a few drips but after adjustment works perfectly.
The AquaSeal, though costly does precisely what it was designed to do. It can take serious punishment form first time users and so far they (4) have not leaked a drop. A good product that I would recommend and use again.