This is the closest one that you posted.
I have several tanks that I need to fill here and there.
I use a 40 gallon rubbermaid trash can for storage, which has a pump and a very long vinyl hose.
I fill a 5 gallon bucket over by the reef aquarium which has a very small pump and tubing going into the sump area.
The small pump is connected to a float switch which is in the sump.
So when the level in the sump drops down a little bit the switch will turn on the small pump and add water to sump.
If you have an in sump skimmer the you will want to have a setup of some kind that will help you keep the water at a certain level. A float switch will help you do this, and your skimmer will skim better.