This type of sponge is invasive in nature, so you may need to trim it to keep it in check. It sends out thin tendrils as it spreads into neighboring corals. Reef-safe-ish.
This long spined urchin is a Diadema Urchin. I recently purchased this for my reef because I find them to be beautiful. They are known to help mow down green hair algae, but urchins often consume coralline algae as well. This one is quite small, as you can tell by the neighboring zoanthids on the left. The yellow thing in the center is not an eye, even though it does seem to be looking around.
This is a harmless Scypha sponge. They are very small, and usually found in our overflow boxes and down in the sump. Attached to rock, glass or acrylic or even your plumbing, they may be about the size of a grain of rice or perhaps larger. These little filter-feeders are reef-safe. Also called Pineapple Sponge, or Q-tip Sponge.
Photo by Debi (dc)
This sponge formed quietly over time in my son's 29g tank. While rearranging the rockwork, I discovered it and turned it toward the front to observe it. I'm not sure if it will continue to thrive now that it has been moved, but it is pretty cool looking. I named this 2" x 2" patch Structured Sponge for lack of a better name. Something to keep in mind when you remove or add rock to your own tank is to make sure the rock stays submerged at all times. When sponges are exposed to air, in general they die and can cause a real problem as they will rot in your tank.
This very cool little creature is a Lettuce Nudibranch. They come in specifically to help mow down nuisance algae such as bryopsis (visible in the foreground). I purchased six of these for my reef tank, and each was about 1" long, and about 3/8" wide. Delicate and very light, they were introduced into the tank with the pumps off. Each one was held gently against the rockwork for about 10 seconds until it held on. Every day, I'd find one or two of them in the sump or refugium, and I move them back up to the display tank. Covering intakes to powerheads would be wise.
A deadly pet. DANGER!! The beautiful Blue Ringed Octopus - Hapalochlaena spp. - is not recommended as an inhabitant for any reef tank. Their venomous bite can cause paralysis (and asphyxiation) within mere minutes, and the animal should not be handled. If one is going to be kept, a purpose built tank with a well-sealed weighted lid is a must to keep this creature from escaping - a recommendation for all octopus, venomous or not. They are not intentionally aggressive toward humans, still care must be taken to avoid being bitten.