Just a few ideas to think about when planning a FOWLR set-up. This is not the only way to set-up a great FOWLR aquarium, but it is my preferred way based on my experiences
Start with the largest aquarium size that you can reasonably get. The reason why I make this recommendation is that some of the most colorful and active fish best suited for FOWLR set-ups (such as triggers, lionfish, and angels), can get big. You don’t want to limit yourself latter on and have to buy another set-up. Although you can have a very small FOWLR tank with a lot of the smaller fish commonly available in the trade, I would still suggest a 4 foot tank at least 120 gallons would be a good starting point. A 6 foot long 180 gallon would be best. I always like to keep my options open for the future.
Although your live rock will supply very effective biological filtration, you will have to watch your parameters closely and consider adding additional filtration if you are having difficulties. Starting off with a sump filled with live rock, a skimmer, and even some macro algae would help a lot here.
Choose your substrate based on the need of any fish you may want to keep.
Pick very good quality rock to use in the set-up. You want to get the most out of your filtration so do not cut corners on your live rock
Pick a very good quality skimmer. A good quality skimmer is the key to getting a keeping a very healthy set-up without adding any extra work. You need the top of line skimmer, but don’t want to cut corner either
Don’t go overboard on lighting. As there will be no corals, pick lower lighting levels and color tone that will allow the fish to look their best. I would suggestion to use mostly acintic and Fuji purple in a FOWLR set-up with a little light ranging in the 20,000 to 22,000K spectrum .
Have a plan for adding your fish. If the fish you want are very territorial or potentially very aggressive, add them all at once and when they are all the same size (preferably very small as well). Keep a very close eye on your parameters as this approach can be risky. If your fish are not territorial and/or aggressive, add them one fish at a time waiting about 2 weeks in between each fish. Going as slow as practical here is the key when you have that option.