Is there somewhere safe to store your belongings while you swim?
Yes. There is a unit just outside of the entrance that store your bags for free with staff looking after it. It is not locked though. Inside the entrance you can pay at the gate for a locker and key. Very small but phones and purses etc. Can be put in them. This is €6 but you get 3 back after returning the key.
Food is not allowed to be taken on site.
There are saftey boxes outside the area.
We did not see anything when we were there, but we just brought a bag for our things. You are very close to your belongings anyway and the area is closed of for people from the "outside". There is a kiosk there, and I saw one of the others that answered stated they have lockers, etc.
I went there in January 2018. I did not see and lockers, change rooms or storage possibilities. I cannot imagine where these could have been. But maybe during (high) season there are more possibilities.
On their website there is a contact e-mail. You can drop them your questions.
Yes, there are lockers and changing rooms.
Get quick answers from Fonts de l'Algar staff and past visitors.