Not all invertebrates are "reef safe". Many of them will tear up corals, consume fish and otherwise destroy other tank inhabitants. The invertebrates listed below are some (by all means not all) generally considered by experienced reef tank keepers to be safe for reef tanks. Keep in mind that nothing is "foolproof" when it comes to living animals. There are renegades in any species, so if your new tank critter doesn't work out, remove it before it can do serious damage.
40 gallon long saltwater reef fish tank - YouTube
Unsurprisingly my plans surround fish. I'm happy with the corals I have now so I don't see myself adding many more. If anything I can see myself removing some. To me reefs with large colonies look so much more natural. Ralph Prehn's is probably the best example of this point. Fish-wise I feel I'm nearly where I want to be. However, deep down I know I'll never be fully satisfied and that can only mean one thing: a bigger tank!
how many fish can i keep in a 20gallons tank? - The Reef Tank
Various businesses have commercialized fishkeeping. With the advent of large scale business operations focusing on breeding massive quantities of specimens, marine fishkeeping has become much more widespread than ever before. Perhaps the biggest disincentive to marine fishkeeping, in comparison to freshwater, is the initial setup cost. A 100 US gallons (380 L) reef tank full of coral and equipment can cost in excess of $2,500 , although a budget-minded home hobbyist could spend less than half of this and still get a satisfactory result.
Step by Step Reef Aquarium - Live Aquaria