1.25 mm Monofilament Tennis String RPM Hurricane - Yellow

Moderate tension (18 kg to 23 kg) will give you more comfort, spin and power. However, the trade-off is less control and precision.
High tension (>23 kg) will give you more precision and control, but you'll need to be wary of tendinitis. You'll also give up spin and power.

Decathlon recommends a tension of 19 kg to 22 kg for monofilament strings. We strongly discourage using monofilament strings for children under age 15 to prevent injury. These strings are best for players who break their strings in under 10 hours of play because they quickly lose their tension.

For multifilament strings, Decathlon recommends a maximum tension of 25 kg. These strings are recommended for players who are prone to tendinitis, for young players and for players who rarely break their strings. Multifilament strings retain their tension well.

Your racket's performance depends equally on the frame and strings! Be sure to choose the right strings.
For frequent string breakers: change your strings when you break them.
For those who never or rarely break their strings, Decathlon recommends changing your strings based on how frequently you play. For example: If you play once a week, change your strings once a year. If you play three times a week, change your strings three times a year.

You should never tighten a loose string pattern - it will have an outsize effect on performance. Looser strings let players play a more relaxed game without forcing to give them more power. As you play, the strings will wear out, even if they don't break. They lose their elasticity and initial properties. The gauge (diameter) also affects the feel. The smaller the gauge, the more power and spin you'll have. The larger the gauge, the more robust (and harder on your arm) the strings will be.