There are all sorts of fittings and connectors available for the standard 6mm airline.
Non-Return Valve or Check Valve - a one-way valve to make sure air can flow towards the tank, but water can't back-siphon from the tank if the pump is turned off or you have a power cut.
Straight Connectors - joining pieces to connect two pieces of hose, e.g. to extend a length of hose.
Elbow Connectors - 90o joining pieces to connect two pieces of hose, ideal for using at the point the airline goes over the edge of the tank to stop it kinking.
Y-Piece - a splitter to let you run two air features off one pump outlet, or to merge two outlets into one for a more powerful bubble-effect.
Regulator Taps - to let you reduce the air flow if the bubbles are coming through too fast, e.g. if you have an air-action treasure chest ornament with a lid that is held open by the force of the bubbles rather than opening and closing gently as it should.