Hi Anthony
Well I'm sure you want to set up a real nice marine tank for fish with live rock and so on, so here?s some help and ideas for that:
Your tank is a cube or almost a cube, which is a very nice shape for a tank because among many good things it allows you to prepare real nice rockscaping displays such as this one from Susan2626
New Photos Of My Tank (
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So before anything else, I would get immediately or as soon as possible, but before anything, 10 to 15% of its volume in Live rock.
Since your tank holds 40 gallons, this means you would get between 33 and 44 pounds of live rock, or somewhere in between that volume.
Once you get the Live Rock, be sure not to let it exposed to the air more that 15 minutes, so regard it as a live animal, which in fact it is only it is millions of them
and put it inside your tank. You don't have to get it all at once and put it at once, you can add it progressively but remember to do it before you put any more fish.
Live rock will be the base of your biological processing. In short live rock is called so because it holds millions of organisms both on its outer surface as well as inner surface, that promote the vital life support biological processing, biological filtering, which combined with the action of the skimmer, and the bacteria in the sand bed, will make your tank good to hold the marine life like the fishes. This combination is called the
Berlin System
Later we can have a look at your skimmer, since as you mention that you want a FOWLR, a skimmer capable of filtering some 3 or 4 times the volume of your tank, will allow you to keep your fish a lot better.
BTW bio balls, sponge filters, and similar materials are detrimental in marine systems as instead of promoting biological filtering, they turn very rapidly into nitrate and phosphate "factories", two chemicals compounds that are very detrimental to the marine environment unlike for fresh water environment. In fact most of the failures in marine systems, or thought to be then marine systems some 20 years ago, were derived by the lack of understanding of this reality. In freshwater, nitrates and phosphates are a major part of the biological system well being, as it is a water biotope that by its nature deals with a lot of nitrates and phosphates. In marine systems is the other way arround due to the nature of marine biotopes, so if you create conditions that do not promote a marine water free of those nitrates and phosphates, you will be poisoning it. This is e very simple and short explanation, but it gives the idea of what could happen.
And there is more if you want to know. For now I leave you this and the suggestion of not adding more fish until you have enough live rock and skimming
Cheers
Pedro Nuno;-)