Im planning to use it for hydroponic. Do i need to use anything else, like vitamins, minerals etc, or is it enough just to use this products?

Maxigro and water is all you need. A Total Dissolved Solids meter (try for 600 ppm) would be useful, but not necessary as about 2 Tablespoons per 5 gallons should get you a reasonable mixture. It isn't rocket science so don't obsess about exactness, have fun. :)

Will it harm fish or turtles?

I have not added fish to the system, I don't think fish or turtles would be a good mix with Maxi Gro

Is the product supposed to be hard as a rock? seems like moisture got in the bag and it’s solid like a rock.

My bag is a fine powder with some chunks.

Can I use this for growing outside pepper plants?

I use maxi Gro on my outside peppers and tomatoes that I grow in 5 gallon buckets with gromix.I have a 150 gallon plastic tank with an electric transfer pump with a 100 foot hose with a nozzle. Works great. I water with it once a week.