String tension refers to the amount of give a string has when engaged by the player; in other words, how hard it is for the player to pull the string. Higher-tension harps mean the strings are harder to pull (less give) and lower-tension harps are easier to pull (more give).
String tension varies widely between different makers and models of harps. There is no right or wrong string tension, only the best tension for the individual player and their needs, the type of music played, and the harp maker’s preference. Pedal harps tend to have higher tension than most lever harps, due to the size and style of playing. Some lever harps have lighter string tension for particular playing styles; for example, Paraguayan harps have very light string tension, which is required for that playing style.
The string material is also part of the string tension equation. There are primarily three different types of strings for lever harps.
Nylon strings are the most common type of string and the most familiar string among harpists. The overall string tension can vary widely in nylon-strung harps, based on the design of the harp itself. Nylon strings tend to be the easiest on the player’s fingers.
Fluorocarbon strings are synthetic and are generally considered to give a somewhat warmer tone. They are medium-to-high tension in nature. Fluorocarbon strings are stiffer and denser than nylon, and are harder on the player’s fingers.
Gut strings are the standard in pedal harps, and have been making their way into the lever harp world. They are generally higher-tensioned than the other strings, and give a warmer, more mellow tone. They also have a different feel than the other two strings, as gut is a natural material, rather than synthetic.
Note: Never change the strings on your harp to a different type of string without consulting your harp maker first. Changing the tension on your harp could affect the structural integrity, and cause damage.
The best way to see what type of strings you like is to try them. Make sure that you try the same type of strings on different harp makes and models, so you are not just evaluating the harp itself, but how the strings compare between models. When in doubt, go with your harp maker’s suggestion on the strings that work best for that particular harp.