[Sk8r]

1. under-oxygenation. Your skimmer, your sump downflow, your return pump, and the cheato in your fuge are the 4 great oxygenators for your tank. The fewer of these you have in operation, the fewer fish you ought to have per gallon of room, for safety. Particularly dangerous: overheating combined with overcrowding. This can take out fish at night, when the oxygen production is low.

2. ammonia. One of the decay byproducts. Barely tolerated by anything, really nasty with stony corals. Avoid using Windex next to your tank---put it on a rag in the next room: that's how much to avoid it. Have a bottle of Amquel [ammonia remover] as an emergency kit is not a bad thing, especially when you're new.

3. caulerpa in the tank with a chancy lighting schedule. The stuff can 'spore', disintegrate without warning and turn your water unbreathable, killing everything in the tank. Light changes seem to trigger this. Fairly rare, but bad.

4. miracle cures. NO med is reefsafe, period. It kills SOMETHING, or you wouldn't be putting it in your tank to kill, yes, bacteria. Well, what recycles stuff in your live sand and expensive rock? Yes, bacteria. Bad. Very bad news. If you're treating something, get it to quarantine to treat, or just write it off. And sad to say, if it gets much sicker, you may be able to catch it. It sounds cruel, but so is it cruel to lose a whole tank and kill all the fish via tank crash.

5. exotic invertebrates. If it's fleshy and weird [cucumber, sea apple, etc] and doesn't move fast---it still has defenses against predators. Toxins [poisons, including extremely lethal ones] are number one. It doesn't know it's in a closed system that will make it breathe the poison as well. It only knows it's stressed, so it'll produce MORE toxin. Everything dies. Absolutely everything.

6. heat. Believing a heater thermostat or trusting just one thermometer is the primrose path to tank nuke. Use more than one, and DON"T trust the thermostat setting without crosschecking. Most are wrong.

7. the unresearched meat-eater. Corals may be on its menu. SO may its tankmates. And it will be really impossible to catch. Guaranteed. [I had a 'reef-safe' eel reputed to be bad about never eating in captivity take out 300.00 worth of fish in 3 days. I had to unbuild my tank to get him out. Naturally he ate my favorite fish first and didn't touch the damsels.]

9. a stirred sandbed: this is very dangerous. Certainly don't do it intentionally. I don't ordinarily recommend buying critters to do 'jobs' in the tank [algae, etc.] but sandbeds---yes. Get some nassarius snails, etc., maybe a fighting conch. Stick to snails for this job, no starfish, no sifting fish unless you have a huge, huge tank. Let Nassarius do it inch at a time. [they burrow]. Don't mess with the sand otherwise.

10. guessing about dosage or dosing without testing. Measure. Remember that most doses of alk buffer or of calcium take 24 hours to percolate through the chemistry. Piling dose on dose is dangerous. Follow instructions and Never Dose What You Haven't Tested For.

11. unguarded powerheads...especially with anemones or soft crawlers in the tank. They follow currents---they will get sucked in. Put a sponge cover on and clean that sponge weekly; take it out for a thorough cleaning in vinegar and put in a new sponge. Or use blue-white filter floss with a rubber band and toss it for a new snip on Saturday morning. Or build a rock 'cage' around it, if you can do it without losing flow. Just don't give them a chance.

HTH, dear friends. Better a word beforehand than a fix after.