Well its your tank and your choice.
A few pointers if I may:
1. With the tank only 3/4 full you no doubt will have much in the way of salt spray and perhaps you will be losing salintiy which should be maintained at 1.025 or 35ppm. I would keep it full and top off with RO/Di water. Keeping it full will add water volume and make things more stable.
2. Having different species of anemones in one tank can be hazerdous, if they meet they could sting each other to death. Lighting is very important to this photosynthetic animal. For the least amount of lighting you should have 4+ watts per gallon of PC, VHO or even MH lighting. Normal ouput is no good for corals.
3. As suggested a synthetic sea salt can be purchased, and by adding it to Ro/Di water to the proper salinity, mixed for at least 24 hours with powerhead and heater to match the temp of tank, can be added. It is usually suggested to do regular water changes per week or two weeks to help maintain water quality, trace elements, and improve the overall system.
4. Depending on where the natual sea water came from it could be polluted, unless its something like that Catalina water. Just taking it off shore isn't good enough.
Good luck and let us know if there is anything else.
Thomas