It's made out of fibreglassyes You can
but I would mix saltwater in a separate vessel
new SW should be ideal….temp, salinity, pH
caviat
I can’t tell the materials used for the construction of the pictured tank
How would you carry out a water change on a tank with such a big volume?With the right toys, any glass box can be a marine tank. You just need proper lights, filtration, and flow to make it.
Like said, saltwater should be made outside the tank to get it to mix right.
If I were to setup a tank of that size shown above I would probably make it a fowlr.With a reef tank, you generally aren’t doing large water changes - I really only do 3-6 gallons each time I do my 40B. Between corals, algae, and a slimmer if you have one, most nutrients/pollutants are used up.
A fowlr with messy predators may be a different story though.
Most with large tanks will usually have a large amount of premixed saltwater for water changes.
How large is the tank?If I were to setup a tank of that size shown above I would probably make it a fowlr.