Nitrox (additional cost per dive) 8€

Nitrox is a gas mixture different to what you usually breathe when diving; it is air enriched with higher oxygen content.
At shallow depths (less than 40 metres) it gives you more no-decompression bottom time, or, for the same bottom time, allows you to have a reduced nitrogen load, and thus be safer and less tired.

Diving with Nitrox has many benefits and requires specific training.