There are formulations for preparing artificial seawater, it is easy to find in internet. A classic formula is available at Kerster et al., 1967.

http://onlinelibrary.wiley.com/doi/10.4319/lo.1967.12.1.0176/abstract

http://onlinelibrary.wiley.com/doi/10.4319/lo.1967.12.1.0176/abstract

You can also buy a mix at your nearby pet store.

You can prepare it yourself or you may buy a commercial mix. There may be different qualities available and its advisable that you check if a given formulation is good enough for your purposes before performing your experiment