Cândida Mendes Sir you can go for YSI Pro DSS Multiparameter Digital water quality meter. Since I have used it in my lab. It is cost-effective and Its sensitivity is very good. parameters; temp, DO, conductivity, Ammonia, Nitrate, etc.

https://www.ysi.com/prodss

I also use the YSI Pro for the field parameters temperature, dissolved oxygen, pH, conductivity, ORP. However, avoid the YSI nitrate probe and use a different system. I have the Hanna portable spectraphotometer for nitrate and it's functional although sensitive to user technique in the reagent mixing.

Be aware of the limitations for each method before spending money on equipment. Good luck.

Thank you all for the help.

You can measure oxygenation using bright steel bars, and pH and salinity using meters