What you are missing is my point.
Moray eels are a true eel (of the order Anquilliformes). In the wild they are omnivorous scavengers that tend to be somewhat territorial. You should therefore consider this when regarding how much room they need.
A moray eel holding tank needs to be relatively large. The exact size does however depend on the species you want to keep. The length and width of the aquarium are the two most important factors for moray eels while the aquarium height is of less importance. An aquarium for a small 3 ft (90 cm moray) species need to be at least 4 ft long and as wide as possible. If they feel crowded, moray eel are likely to try to escape. It may also lead to bad water quality as well, if it is truly crowded.
You should decorate your aquarium tank so that the moray eel has at least one or preferably several hiding places. If you keep several morays in the same aquarium it’s important to make sure that each moray can have its own hiding space.
Moray eels usually see other fish as meal if they are small enough to be eaten by the moray eel.
The water values for a moray should preferable keep a water gravity (salinity) of 1.022 to 1.025 and a temperature of 72 to 86 degrees Fahrenheit (22-30 degrees Celsius). The pH level should be kept above pH 8.0.
Moray eels produce a lot of waste matter which means that you will need good Chemical, Mechanical and Biological filtration going as well a good protein skimmer to keep the water quality up. You will also need to replace 20-25% of the water weekly to keep the water quality at top. (It might be enough to change about 10% of the water but a larger exchange is to be preferred)
What I am trying to get at is that you are not getting the big picture with some of these species. Just because you what you read says they should be compatible, fish that grow to larger sizes should be put into consideration as they grow they will need enough room to not feel limited to the amount of space that is given to them.
So otherwise, some fish will find themselves quickly looking for space, or they will make their own space within the given space you have given them.
That means through elimination of whatever else is in the tank that they can get rid of.
Reef tanks, are not like FO tanks. You compare apples and oranges.
Reef tanks will have enough biological filtation to support livestock. Remember that also with reef tanks, they have sumps, refugiums or something else along those lines to provide extra volume. Smaller fish waste is not like larger fish waste. Even 15 clownfish in a tank will probably not make the amount of waste of an eel or a marine betta.
Also, how are you going to put fish that dwell in rockwork along with a fish that will sleep in the sand or in the rockwork?
Do more research on the fish you are intending on keeping. I see your idea as an expensive recipe for disaster.
