More photomicrography
Here's a tour through the slide..
And some of the captures:
For some reason, there were a LOT of vorticellas in this sample. I've only ever caught them on "film" in saltwater a couple times. Vorticellas are kind of like a mouth on a string, always filtering, filtering, filtering water.
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Kinda sorta, but not really. A tank will go through waves of explosion and contraction in different populations. Typically it starts with diatoms and dinoflagellates, then cyanobacteria, and usually green film algaes. Kind of like a stop light, but yellow, red, then green.
Assuming you start with a lot of good sources of diversity, you can expect to see explosions in amphipods, asterinas, micro-brittle stars, bristle worms, collonista snails, dorvellid worms, spaghetti worms, isopods, copepods, sponges, mysid shrimp..
Heck, here's a page I put together talking about biodiversity..
http://rowelab.com/fish/bingo.html
Thing is, two tanks set up with the same initial bioload can end up with different 'apex' organisms. It's a bit of a crap shoot.
"Live rock" is porous limestone that's quarried (usually from ancient sea beds!) then seeded with life, either in a facility or in the sea. Aquacultured live rock can contain hitchhikers like mantis shrimp (fish, invertebrate, and/or snail eaters) or "gorilla crabs" (which also eat other things you want to keep.)
Those two are the biggest (size-wise) hitchhikers that folks usually run into with aquacultured live rock.
There are other, microscopic and near-microscopic pests that you can get- from ciranolid isopods that are parasites on fish, to marine 'ick' (cryptocaryon irritans), or parasites of coral like AEFW (acro eating flat worms), even pest anemones like majano or aiptasia.
This was a piece of uncured rock from a local store- notice the sponges, bristle worms, and yes, a couple of aiptasia anemones.
The way to balance the scales is to get uncured live rock, fresh from the distributor to maximize diversity, but house it in a quarantine system and set traps for things like crabs and shrimp, while letting the obligate parasites starve without a host.
I used to go to the Texas coast about once a year and pick up some sand / mud / algae / pipefish / fish to throw into my systems.
They add a great local flair, but they're not as colorful as other marine critters. Maybe one day in the future, I'll set up a Texas coast tank, limited to Texas gulf species.
You can *usually* tell by the end of their claws. Are they packing salad tongs or steak knives..
These claws have a flat pad at the end, used to pick algae. So this guy is primarily* a herbivore.
This little guy is primarily* a meat eater, based on the scissor-like claw meant for cutting and tearing.
Come at me, Bro!
* Almost all crabs are opportunistic omnivores, and some hobbyists believe they're all evil, eventually to go on a killing spree. 
opportunistic omnivores. 
Learn something new every day! Now I know more about crabs.
