Here's something for you all ,I ran a 80 litre marine tank ,successfully might I add ,with anemone ,fish ,and corals for 3 years using salted deionised water ,I mixed my own water and used marine salt(obviously) to 1020 on the hydrometer.i checked the water every 2 weeks with water testing never added any ro water just deionised /salt mix to top up with and deionised water is a fraction of the cost ,(DO NOT MIX ) with salted RO water it's one or the other.
Now I'm not saying let's all go out and change over to save money but I tried it and had no problems and in all the 15 years of marine fish/coral/anemone keeping I've never used a protein skimmer either ,just live rock,live sand,external filter and keeping an eye on water parameters every 2 weeks and adjusting accordingly.
So yes skimmers have benefits but they're not a necessary bit of kit to keep marine fish which can again save you money from buying one , and having a bulky bit of plastic in the tank .just thought I'd put that bit of info out there to you all .now I'll wait for all the you can't do that or this and you need this or that ,lol
My answer is I've done it and it works so have some of them apples 🍎 marine nerds . Lol 😝