Restriction is anything that impedes flow, thereby reducing the amount of energy in the water - and it happens to some degree or another any time anything is in the way of the flow of open water. For instance, if you have a 90 degree elbow in your loop, the water is rushing towards it and slams into the wall of it before diverting its path. This "slows the water down" - it impedes its natural flow.
How? Remember your laws of inertia, and think of it like this: as a water molecule moves in the pipe, it wants to continue to go the way it already is. Obviously, it doesn't just naturally turn - it continues until it cannot (collides with the elbow wall). This means it has to stop, and then has to start again in a different direction - which it has to borrow energy from its neighbors to do.
Restriction is created by sharp turns, sharp decreases in path volume (i.e., a spacious block crammed back into an 8mm ID hose) and things in the general path of the water. The more restriction there is, the more head pressure from the pump is needed to compensate for it, which means more noise and power.
) about making the tubing a bit neater and in theory I'm looking at using 4 45 degree and 2 90 degree fittings.
