[Sk8r]

Bad things:
1. mantis shrimp in a reef. Kills fish.
2. pistol shrimp in a reef. Ditto, eventually, as they get bigger. The pistol goby pair is very interesting in a small tank, but the tiger pistol in particular is a problem as it grows.
Warning sign: loud sounds from the tank at night. Pops or snaps.
3. eunicid worm: kind of like a centipede with tentacles atop its head.
4. fireworm: has a very 'fluffy' appearance down its sides, bristles that make it really quite beautiful.
5. hairy crabs are particularly bad; crabs with a large claw are bad; in general, the only 'safe' crab is the emerald mithrax; but even they can get a bit nippy with fish fins. In general they don't eat bubble algae---or not very much of it. In my tanks, I think the record is two bubbles and a chunk out of a mandy's finnage. Do not buy a sally lightfoot crab for a reef: they get the size of a dinner plate; do not buy an arrow crab---they eat your bristleworms, and this is a bad thing. But they're neat critters.

6. caulerpa algae in any form: starting from a sump fuge, it reproduces from any fragment, or from spores; it gets through the pump, it roots in your rockwork where nothing can remove it; and it's toxic to eat. Few fish can eat it. It can take a tank, especially a tank too small to have the fish that can eat it...and that fish grows large and kills tankmates when it feels cramped. Not a good thing, no.
7. nudibranchs: sea slugs---no big deal most times, but if you have zoas, they eat those, and they can lay eggs. A tank that is a waystation to your main tank is a good idea (ie, a qt). Dipping is where you start, but corals that come on complex rockwork can deliver surprises. Caution is a good thing.
8. little red bugs that you need a magnifying glass to see: bad news for sps corals, particularly. Dip. And buy from known clean sources or friends whose tanks you know are clean.
9. asterina stars (little starfish always one arm short---) are in both the good and bad column, depending on what you keep. Zoa keepers, as I recall, don't like them. They've never given me any trouble.
10. little majano or aiptasia anemones: minor problem, usually. I always keep a couple of peppermint shrimp, since I seem to grow aiptasia in my fuge. One of my peps is getting very large and fat. But I keep sps and lps corals with no fuss. I've been known to move a torch coral next to a persistent stand of these stinging creatures: torch is just as nasty in that department.
11. tiny little brown fuzz that, if magnified, looks like trees with branches: hydroids. They sting, too. If you have a rock with hydroids, one trick is to coat the outbreak with reef putty, or remove the rock. I've also had some success just flipping the rock and burying the colony in the sandbed.
12. little things that look like the Star Trek symbol...triangular with a forked tail: flatworms. Consult here before you try removal and we'll walk you through it safely. These are not a good critter for your tank. Some flatworms also appear in large economy size, but I've never seen one of those: most are quite tiny.
13. a little crustacean with black alien eyes (you know, the Roswell aliens) ---cirolanid isopod: parasite on fish.
14. white saltlike dots on your fish that aren't sand grains that fall off again: these are pimples produced by the ich parasite, and no, a cleaner shrimp or goby can't cure it. This is a nearly invisible [near-microscopic] parasite that lives in your sandbed, and this is why you use quarantine in a bare glass tank, to be sure this parasite can't reach your main sandbed.


Good things to find: worms, bristleworms, spaghetti worms, shell-less snails [stomatella], various small corals and buttons, sponges, in general---sponges are hard to grow, in most tanks, so an outbreak of little white pineapple sponges tends to be brief. Same with little brittle stars and a plague of asterinas---as soon as they consume whatever they have in excess, they die back.
The meme has it that worms are good unless you have 'too many.' Well, that's not true. WOrms in just about any number short of heaving masses are good because they're eating something your tank has too much of. They and your skimmer are cleaning up the excess in your tank, and if you get something to kill the worms you'll still have the excess. Let them work, in general, until they've done their job. Then they'll gradually die back and you won't have many worms. Understand they don't have jaws, don't really even have mouths, just an opening with suction that takes in food: all they can eat is slime or dissolving food pellet, particle at a time. They don't bother corals, they don't bother fish (if your clown gets a faceful of spines from them, it's ok: the fish will learn after a while and stop trying to use the worm's burrow. The spines fall off harmlessly after about 3 days unless he's getting a nightly dose.) Just treasure your worms. The two exceptions are above. There are some that get about a foot long: these usually inhabit a particular rock and only stretch out to get food, then retreat rapidly.
Little white bugs are good: these are copepods and tiny fish love them. Dragonets have to have them.
Little funny crustacean swimmers are good: amphipods.
In general, most that comes in with our rock is beneficial. Accept it. It's why I don't 'cook' live rock that I get that's full of life. I want whatever it can bring in---and on average, it's really good, interesting stuff. Some people want to 'control' their tanks---that's ok; but I prefer the wild and wooly rock, so to speak. Two different strokes for two different folks. Both work.