In Saltwater tanks rock is a very common decoration. Besides looking very natural it also adds other benefits. Porous rock with lots of holes helps to add more area for bacteria to live on. Rocks can be stacked to make caves and tunnels for fish to hide in. If you should decide to get corals one day rocks are perfect to mount them on.
The short answer is that while they are helpful and provide lots of uses, no, they are not needed. In a fish only (FO) tank there is no rock. It might have other decorations and probably sand or gravel but that’s it. The perks here are a full tank of swimming area for the fish and an unobstructed view at all times. Unless you add decorations though, the fish will not have any place to hide when they feel threatened.
Turning that into a fish only with live rock (FOWLR) tank is easy. Just add some rocks! There are different types of rocks you can add but they all fall into two simple categories: live or dead.
Dead rock is a dry rock with no active bacteria on it. It usually looks white in most cases though some come colored to mimic coralline algae. It often comes “uncured” and may be full of nutrients like phosphate.
Live rock is a rock that has been in saltwater for a while and has been allowed to grow a bacteria population on it. It is often already cured and had most nutrients leached out but that’s not always the case. Live rock will cost more than dry rock because it takes time to create as well as the fact that rock absorbs water, making it heavier. 40 lbs of dry rock will be larger than 40 lbs of wet rock.
I strongly recommend adding rock to any tank. It can create a natural setting for the fish and a very unique look to the tank while providing lots of good surface area for bacteria to thrive. Adding live rock to a tank is a great way to speed up or, even better, skip the tank cycle. One such place to purchase rock from is Tampa Bay Saltwater. They offer live rock at an affordable price and ship it to you! This rock is placed in the ocean and allowed time to gain life before it gets pulled and shipped. This means you get some amazing amounts of life on these rocks. Not just bacteria but algae, snails, starfish and shrimp are just a few of the hitchhikers that come in with this rock. They can be found at https://tbsaltwater.com/index.html