Which magnets can be used in water for a long time?

This depends on the material of magnets. Not every kind of magnet can be used in water, because a rusty and corrosive magnet may harm aquatic life. It is reported that samarium cobalt and ferrite can be used normally in water. Neodymium iron boron and aluminum nickel cobalt are not recommended to be used in water for a long time, but they can also be used in water. For example, it is often seen that some people use a slapstick magnet, which is neodymium iron boron magnet.

Generally, the magnets used in aquarium are mostly pump rotors. If you have a pump in your cylinder, its magnetic field is strong enough to be much stronger than the magnets, because the pumps rely on the coil to generate strong magnetic field to make the rotor rotate at high speed to pump water, and the rotors are permanent magnets. I don't believe you can dismantle the rotor and put it on the iron to see if it can be absorbed.