Whenever you buy new livestock, you can place it in a QT (Quarantine Tank) or put it directly into your DT (Display Tank). The choice, of course, is yours, and it's important to understand that there are pros & cons no matter what you decide. We live in a day and age when everyone is looking for a "perfect answer" - although no such thing actually exists. 
First, there is much debate on exactly how to QT in the first place:
However, if you are always on the go, busy/hectic schedule or just don't notice the little things; then it would be wise to implement some form of prophylaxis when it comes to diseases. I recommend using Prazipro (or API General Cure) in order to deworm, and either copper or Chloroquine phosphate to treat for ich/velvet.
Unfortunately, quarantining just your fish is not enough. Parasite tomonts can encyst to corals, shells, any hard surface really. More info regarding How to Quarantine Corals and Inverts can be found here: https://www.reef2reef.com/ams/how-to-quarantine-coral-and-inverts.228/
And also here: https://www.reef2reef.com/ams/coral-invert-quarantine-time-frames.382/
Is quarantining foolproof? Of course not! Can it fail? Yup! What are the reasons why? Well, first and foremost is human error:
There are also a number of factors beyond your control which can cause QT to fail:
You wouldn't bring a beloved pet dog or cat around another that had fleas, mange or rabies, would you? Hoping he/she would build up a resistance or immunity to it. Don't our pet fish deserve equal protection? That right there is why, at least for me, the pros of QT will always outweigh the cons.
First, there is much debate on exactly how to QT in the first place:
- Some choose to prophylactically treat for parasites, worms, infections, etc. by using medications/chemicals before symptoms of disease even present themselves. The argument here is by the time symptoms do show it is sometimes too late to treat/save the fish. The downside to this approach is all meds/chemicals have side effects and thus, can be harmful to fish (no way of sugarcoating it).
- Some choose to passively observe (sometimes in a mini-DT like natural setting), and only treat if symptoms are observed. This is easier on the fish, but one must react quickly if white spots (for example) suddenly appear. If using rock/sand in this observation tank, it is advisable to have a second QT on standby which can be setup on short notice. This "treatment tank" should be bare bottom (with no rock) to avoid absorption issues with medications. You can use PVC elbows (found at Home Depot or Lowe's) as hiding places for the fish.
However, if you are always on the go, busy/hectic schedule or just don't notice the little things; then it would be wise to implement some form of prophylaxis when it comes to diseases. I recommend using Prazipro (or API General Cure) in order to deworm, and either copper or Chloroquine phosphate to treat for ich/velvet.Unfortunately, quarantining just your fish is not enough.
Parasite tomonts can encyst to corals, shells, any hard surface really. More info regarding How to Quarantine Corals and Inverts can be found here: https://www.reef2reef.com/ams/how-to-quarantine-coral-and-inverts.228/And also here: https://www.reef2reef.com/ams/coral-invert-quarantine-time-frames.382/
Is quarantining foolproof? Of course not! Can it fail? Yup! What are the reasons why? Well, first and foremost is human error:
- Not checking your copper level on a regular basis to ensure it is remaining therapeutic. More info on that here: https://www.reef2reef.com/threads/copper-test-kits.257924/
- Dosing non-copper medications into a long-term QT and/or one with rock & sand. Look here: https://www.reef2reef.com/threads/qt-and-biofilm.292878/
- Failing to take steps to avoid cross contamination and not respecting aerosol transmission: https://www.reef2reef.com/threads/aerosol-transmission.190292/
- Copper and other meds are immunosuppressants; so it's very important to feed nutritious, vitamin enriched foods during the QT period (and beyond!): https://www.reef2reef.com/threads/vitamins-herbal-remedies.304287/
- Knowing copper is a poison, it's a good idea to slowly ramp it up to therapeutic over a 5-7 day period. This allows your fish in QT to slowly adapt to it, and there should be warning signs of copper intolerance (e.g. fish stops eating) before the fish just dies from it.
- Allowing ammonia to build-up and kill your fish in QT. Remember, using any ammonia reducer (e.g. Prime, Amquel) with any medication (especially copper) is a BIG NO-NO. Also, since most medications in the water will cause false positives with liquid ammonia test kits, it's wise to always use a Seachem ammonia alert badge in QT: https://www.seachem.com/ammonia-alert.php
- Mixing/combining medications in QT is never a good idea unless faced with an emergency situation (i.e. treating multiple diseases). In addition to depleting available oxygen out of the water, the fish's liver & kidneys has to process/filter every med you dose into the water. Also, more meds dumped on a fish causes increased side effects like appetite suppression. BOTTOM LINE IS MOST MEDS/CHEMICALS ARE BAD FOR FISH, and should be viewed/respected as the "lesser of two evils" when faced with parasites, worms and flesh eating bacteria. ;Yuck
There are also a number of factors beyond your control which can cause QT to fail:
- When you go to the doctor, the treatment or medication he/she prescribes doesn't always work. Same thing can happen with fish! Since April, I've encountered half a dozen strains of praziquantel resistant flukes & turbellarians (Black Ich) - that I had to use hyposalinity or formalin on in order to fully eradicate.
- I'm encountering many more occurrences of Brooklynella & Uronema than I expected, and on fish previously not known to be susceptible to these parasites. Therefore, those who choose to prophylactically treat may want to use metronidazole as a precaution. Chloroquine phosphate and formalin are two other options for dealing with brook/uronema.
- It is theoretically possible (but never confirmed) for parasites (and other pathogens) to develop a resistance to medications (or copper), if they have been previously exposed to such at a subtherapeutic level. We've all heard stories about how LFS and wholesalers like to use subtherapeutic copper to "control" parasites. Are they unwittingly creating fish superbugs? :mad:
- Just like with humans, things can go wrong inside a fish as well. With internal parasites/intestinal worms you may notice white stringy poo, but symptoms of internal infections are almost never visible. There are no viable treatments for viruses (e.g. Lymphocystis) in SW fish - the best you can do is manage symptoms and hope the fish's natural immune system will send the virus into remission. Spinal injuries/swim bladder disorders can also sometimes prove to be fatal in QT. However, it's important to note that all of these are possible threats you face whether the fish is in QT or not.
- ^^ Knowing all of the above, don't be in a rush to get your fish out of QT! There should always be an "observation period" of at least 2 weeks after you use meds on a fish, to ensure nothing was missed (or returns.) This observation period should take place in nonmedicated water so there is no chance of a disease just being suppressed.
- It's no secret that the larger your aquarium is the more successful you are likely to be at disease management; since parasites, worms & harmful bacteria are naturally diluted by greater water volume. The same holds true for tanks lightly stocked with fish (reduces the pathogen's food source) and/or fish with thick slime/mucous coats (e.g. wrasses, clownfish, dragonets) which serves to protect them. Conversely, Acanthurus Tangs and other thin slime coat species are most at risk from parasites, worms, etc.
- A UV sterilizer and/or diatom filter is useful for siphoning parasite free swimmers out of the water, albeit probably not all of them. The use of dissolved ozone can help fight disease, and an Oxydator has been likened to, "putting an oxygen mask on the fish" to aid with breathing when parasites invade the gills.
- Feeding live foods, utilizing proper nutrition and using vitamins can greatly help to boost a fish's natural immune system. In addition, herbal remedies can sometimes be useful to boost the immune system and/or thicken a fish's slime coat to withstand damage inflicted by parasites, worms and bacteria.
- Maintaining pristine water conditions, stable parameters, and limiting fish conflicts (which sometimes leads to injury/open wounds) helps to keep "stress" in an aquarium at a minimal. Poor water quality, fluctuating parameters and constant fighting lowers a fish's immune system, which makes him more susceptible to parasitic infestation.
- Sounds good! So what's the problem with just doing that?? Well, a) It doesn't always work b) It's a forever thing. If you start using a UV sterilizer to manage parasites in your aquarium, change the lamp religiously every 6 months and have a backup on standby just in case. Because if it suddenly goes down, your entire fish population could be next. If you use herbal products to control parasites and secondary infections in your DT, buy stock in the company because you will be dosing that stuff for life. Oh, and when you go on vacation be sure to hire a tank sitter who can manage all these little idiosyncrasies or you might come home to a disaster... For me anyway, there’s enough to do in a reef aquarium on a daily basis without adding “battle fish parasites” to the list.
You wouldn't bring a beloved pet dog or cat around another that had fleas, mange or rabies, would you? Hoping he/she would build up a resistance or immunity to it. Don't our pet fish deserve equal protection? That right there is why, at least for me, the pros of QT will always outweigh the cons. 
