The most common cause of loss or marine life is declining water quality, mainly due to biological cycling/filtration "misunderstandings". Other category headings of toxic source include decor items, cleaners/other household poisons, among many other biological sources.
Fret not; ninety-nine percent plus of these "problems" are iatrogenic (caused by aquarists themselves), are easily avoidable, and will be made simple to understand here.
What you "have to do" to prevent a toxic water condition, indeed to be a successful marine aquarist period, is more a matter of what not to do. You don't need fancy instrumentation, expensive, complicated filtration, a zillion dollars and tons of time to dedicate to monitor and fool with your system. These are more often the root of problems than salvation.
What you do require is a broad grasp of how your system works, a conscientious arrangement in setting it up, and maintaining it properly, and to leave it alone. Due to inaccuracies in test gear, toxicity of treatment modes, other controllable means of preventing loss of water quality, you are encouraged to "take ten (or more) deep breaths", consult reference people and printed works, and "run around the block" before making knee-jerk reactions, buying and putting "something" into your system. I realize that we live in a "market-driven" society; that we're trained from early childhood to be "good" consumers (i.e. to buy things, to treat symptoms, real and imagined), but marine aquaria are not improved with general tampering.
Though it seems easier to point the finger at "cyanide collecting", "reef environmental degradation", poor tap water quality... anything but ourselves; the fact is most marine livestock isn't "lost" to these or other "outside" causes; ladies and gentlemen, it is killed by hobbyist ignorance and well-intentioned "intervention". This section describes the bulk of such unreasonableness and it's avoidance.
Biological, Filtration, Other Endogenous Sources:
Picture a newspaper, electronic or print, in the year 2025. I can see the same headlines as now; serviceman suffers head injuries in motorcycle fall, government officials caught stealing, marine aquarium fishes dead again from biological filtration snafu...
Too much ammonia and/or nitrogen from not properly stocking, overfeeding, killing off nitrifying microbes has been, is currently, and probably will be for years to come, the primary cause of loss of livestock, toxic poisoning or otherwise. Let me help you help yourself in evading this. Following, in keeping in-stride with current genres, is my 10-step Avoiding Biological Toxicity Problems Program (c):
1) Steady Stocking: given you've set-up your system adequately; whether you're utilizing a bacterial "culture", with or without a feeding supplement, or biological "starter" from another going system's substrate, filter media, or "detritus" (all good ideas that we'll cover in 3) F) iii) Biological Augmentation), you are encouraged to have patience in initially stocking, feeding, aka changing the bio-load of your new system. Four to eight weeks of very few organisms, fed sparingly to establish the rudiments of biological nutrient-cycling are called for. Take your time. Take other writer's accounts of how many inches out and consider instead cubic inches that can/should be allowed. No more than one inch cubed of ammonia/nitrite tolerant life per twenty gallons is a good value for the "run-in" period, and no more than that per week added after.
2) Overfeeding, see 1) above. Much of the foods you'll add have the same disposition if they're consumed or not. They need to be expeditiously converted to innocuous products. Put another way; a gram of proteinaceous food is either A) consumed and converted to ammonia to nitrite to nitrate, or B) not consumed and converted to ammonia to nitrite to nitrate. Older texts even went so far as to suggest throwing in chopped clams, et al. (stinky) without any life in the system, to speed establishing cycling. Don't overfeed.
3) Medications; several common ingredients in "marine aquarium remedies" many antibiotics, Methylene blue, potassium permanganate, and more (get in the habit of reading the bottles if you intend to engage in such voodoo usage) kill beneficial microbes. These should never be used in your system tank. One of the recurring themes in this book is the need for and use of a separate quarantine/treatment/isolation/safety-backup system. Please, please, puh-leeze do not waste your resources on fancy gear, test kits, meters and such without first having an alternate "way-station" for bringing in new livestock, isolating bullies/bullied, treating for infectious pests and parasites outside your main system.
4) Nitrate accumulation. By itself not a concern for most livestock/systems. I have witnessed aquaculture facilities for both fin-fish and invertebrates, with several hundred parts per million total nitrate concentration. The inherent dangers and results from utilizing chemical filtrants (so-called nitrate removers), (anaerobic) denitrators, even algal-scrubbers, are not worth the trouble they cost/cause. Even live-coral systems, by some the most demanding of low-measurable metabolites can/have been adapted to moderate nitrate levels.
What a measure of nitrate can and should do fro the conscientious aquarist is serve as a guide into the total water quality picture of their system. There are many other chemical and physical changes that occur in a small captive system that aquarists are not aware of; and shouldn't want to be. They are all characteristic of the general trend of over-crowding an artificial environment. Your job is to be aware of and resist those changes given the best available, most appropriate technology.
Do have excess filter and circulation capacity, do dilute converted wastes via water changes, do exacerbate conversion with live rock, calcareous substrates... and don't worry.
5) pH Drop we've chatted over under the pH Section. Substrates, calcareous rock, coral skeletons alone will not maintain a pH above 8.0. Biological processes, including filtration conspire to drive pH down, typically 0.05 to 0.1 point per week without intervention.
The easiest and safest ancillary buffering method involves the simple use of baking soda, sodium bicarbonate. This and other alkaline buffers may be purchased; but I assure you, the biggest aquarium service companies use Arm & Hammer (tm). About one teaspoon (5 g) dissolved in water per twenty gallons per week in conjunction with other mandatory maintenance will keep your system's pH right.
6) Wipe Out Syndrome I'll mention in passing, in a feeble attempt at comprehensiveness. There are "mysterious" total losses that have been chronicled, especially in newly set-up and monoculture (one species) systems. Bacterial "wars", toxins, and more have been speculated. Suffice it to say/write that there are such things, that you must keep your eyes on your livestocks' behavior, and that if all seems to be going side-ways, 1) check water quality, 2) either execute a massive water change, or 3) move them to your quarantine/hospital/alternate tank.
One more important (re) note; the use of protein-skimming and "wipe-out syndrome" are mutually exclusive. Once again, a must for your system has got to be a functional foam fractionator.
8) Lack of Oxygen is more common and dangerous than you may believe. Tight fitting tops and motor-driven circulation combine, often with elevated temperature, to reduce the availability while increasing the need for oxygen. Saturation is only about seven parts per million under normal aquarium conditions and this can get used up quickly by something dying unnoticed, over-feeding, microbial population explosion or die-off... Don't rush out and buy a meter with an alarm that will call your pager... Simply install a mechanical aerator (an air-driven airstone). Small (less than one hundred gallon) systems should most probably utilize these for driving their undergravel filters. Venturi-type skimmers really goose oxygen concentration, especially if they're outfitted with an ozonizer.
The real bottom-line is watch your livestock what are their ordinary respiratory et al. behavior/rates? If they're breathing hard, fast, suspect low oxygen tension and correct it by removing some livestock, increasing gas exchange.
8) Anaerobiosis; the effects of "life without oxygen". You're probably familiar with the rotten egg hydrogen sulfide (H2S) smell; dark sand, stinky bubbles from stagnant water areas... You want to avoid these in your marine aquariums. When given the conditions, aerobic conditions and microbes will supplant anaerobes. Move and vacuum accumulating mulm in and around your decor (and filter sump) on your scheduled maintenance schedule. Be careful if/when the power goes off to vent, clean/backwash closed canister filters. They go anaerobic in a matter of minutes and will easily flush undesirable by-products into your system.
9) Avoid "Bad" Foods: What are these? One's your livestock will not or cannot consume/process, and those with limited nutritional value, In particular I'd like to see gelatin-based frozen foods and air-exposed dry-prepared foods rode out of town on a rail.
The latter especially gall me. The best foods in the trade have been very extensively researched; and are prepared and packaged under amazingly exacting conditions. All this effort is for naught if the manufacturer's efforts are thwarted by "re-packaging" by some method that exposes the food to the air. Would you purchase/eat even just cereal that's been thus exposed for months? Not me either; it's neither palatable or nutritious. Why do this to your livestock's food? Don't participate in such foolhardy and self-defeating chicanery, use only factory-sealed foods.
10) Other Biological Hazards: Livestock. The "enemy from within" includes several types of organisms; sea cucumbers, all stinging-celled animals, Spanish dancer swimming snails, live rock, etc. release materials that must be dealt with through filtration/circulation provisions, maintenance... The only legitimate course of action is for the hobbyist to research the particular life-habits of the life they intend to keep, apart and together.
Be aware that all living things are a potential problem in that when they expire (sometimes hard to tell) they can quickly and poison your water. The more living "stuff" concentration-wise, the greater your need for diligence.
Tramp metals, contaminants brought is accidentally as part of your substrate and aquascaping are a slight possibility with naturally collected materials. Iron, in particular may be a problem, from metal cans, crushing/processing tools, shipping... Be suspicious of 'rust' stains from coral sands, gravel and any volcanic rock.
Other soluble minerals that include zinc, copper, arsenic, lead, et al. can get into a system the same way. My experience has been that these are far and away unusual sources of poisoning except for folks that utilize 'rocks' from other than marine origins. Oh sure that geode, quartzite, even petrified wood looks great underwater, but is it worth the risk of poisoning your system?
Some artifacts you have to watch out for too. Coral skeletons, shells, etc. are suspicious for two reasons. have they been cleaned/prepared properly and do they dis-allow anaerobic circulation? Obviously, animal residue and residual cleaners you don't want. Take care to occasionally carefully lift out and rinse cavernous shells.
When in doubt leave it out.
Cleaners & Other Household Poisons:
Ammoniated cleaners, especially window and counter top types, I've seen kill whole systems. These can get into the system through the air, food, your hands, air-supply... Tobacco is another question. Back in my happy-dark days of "lake-maintenance" we'd spray nicotine sulfate (with a spreader-sticker) onto surrounding trees to reduce aphid populations (on water lilies). Any over-spray resulted in massive fish kill. Though there may be little/no documented proof of such, I'd build or buy an in-line air-filter as readily as a check valve; and I wouldn't have an air-pump without a check.
Not to cause hysteria, or paranoia; my advice is first and foremost, try to keep your hands out of the system as much as possible. Next, get a pair of arm-length gloves (like kitchen types) and dedicate them for Aquarium USE Only. Lastly, check out the tong, feeder stick, home-made and store-bought tools available.
Tap (Mains) Water:
I've said my piece concerning the hypocrisy of going overboard with ultra-filtered water. The tap is safe for you to drink, and use in your marine system.
Want to do yourself the most good initial water quality wise, for less money and hassle? Rig up a pre-mix seawater barrel system with a heater and circulation. The salts will complex any amount of other matter in your tap. The benefits of providing completely dissolved, similar pH, temperature and specific gravity synthetic are far greater than anything you might get from utilizing even (flash) distilled water.
Regarding test kits and the need for marine aquarium testing; I'm reminded of a discussion of accuracy and precision. The former is the capability to measure what something really is (compared to an established standard). Precision is the capacity to get the same measure again and again. There are both problems with test gear in our interest, and therefore dangers of both false positives and negatives.
In a study of twenty two test kits for ammonia, nitrite, nitrate, and pH, Bower found only four that "gave reasonably accurate readings". She suggested that hobbyists forego routine water analysis except for weekly determination of temperature and specific gravity.
Though I don't fully agree with this assessment in it's degree of condemnation of the validity of such testing, I am in allision with it's valid intent. You don't need more "tools" than you can make reasonable use of; don't get hung up on the specifics of any one test.
Buy decent organisms and put them in an optimized environment; do simple, routine maintenance including frequent partial water changes, checking temperature and specific gravity weekly. Natural marine habitats are remarkably stable compared with the terrestrial ones we live in or freshwater ones we've kept. Fluctuations in water quality are to be avoided, the least worry of which should be those that we as the fish gods, induce.
Anderson, Frank. 1992. NTS, new tank syndrome may be on it's way to oblivion. FAMA 4/92.
Bower, Carol E. 1980. Saltwater Aquariums, water quality with water analysis. FAMA 5/80.
Greenfield, Dick. 1978. Marine filter bed materials, pt. 2. FAMA 12/78.
Michael, Scott. 1995. Watch that pH; how unexplainable fish deaths may be related to changes in pH. AFM 10/95.
Moe, Martin. 1983. in Editorial reply re toxic tank syndrome, wipe out". FAMA 10/83.
Refano, Joe. 1979. Eliminate mysterious losses. FAMA