TA 990 Power 1.27 mm Monofilament Tennis Strings - Black

Moderate tension (18 kg to 23 kg) offers more comfort, spin, and power. The flip side is that you'll have less control and accuracy.
High tension (> 23 kg) provides greater control and accuracy. It can, however, lead to tendinitis. You will also have less spin and power.

For monofilament strings, Decathlon recommends a tension of between 19 kg and 22 kg. We strongly recommend against using monofilament strings for children under 15 years, to avoid injury. These strings are best for players who break 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 people who are prone to tendinitis, children and youths, and people who don't break their strings often. Multifilament strings keep their tension very well.
For natural gut strings, Decathlon recommends a maximum tension of 23 kg. These strings offer superior comfort, feel and power. They also retain their tension extremely well.

Your racquet's performance depends 50% on your frame and 50% on your strings! Be sure to take the time to choose the right strings.
For players who break their strings a lot: change them as soon as they break.
For people who rarely or never break strings, Decathlon recommends changing them as many times per year as you play per week. E.g. if you play once per week, change your strings once per year. If you play three times per week, change your strings three times per year.

You shouldn't tighten strings that have gone dead since their performance will have been lost. A loosely strung racquet will let you play a more relaxed game, giving you more power without having to force it. Over time strings can become worn without breaking. They lose their elasticity and initial properties. The gauge (or diameter) also affects play. The smaller the gauge, the more power and spin you'll get. A larger gauge will give you stiffer strings with a heavier impact on your body.