We've all heard it said, "Every tank has ich." And sometimes it does almost seem inevitable. 
However, the truth is no new tank comes preinstalled with ich. Cryptocaryon irritans aka "ich" is a ciliate protozoa that is either brought in on an infected fish or from water, rock, coral, etc. that was taken from an ich-infested environment. Let's break down all the different ways a disease can be introduced into your DT:
All tanks start out disease-free, unless it is "inherited" by using rock, sand, water, equipment, etc. from an already infected system. It must be "introduced" in some way - just like pods won't magically appear in your tank without a source (rock, chaeto, bottle of pods).
What about bacterial infections?
When you cut yourself, you apply a topical antiseptic or antibiotic to ward off the possibility of infection. Unfortunately, this isn't a very practical thing to do when a fish gets cut, stung or otherwise damaged. (Although there is a product called Bio-Bandage for just this purpose.) Unfortunately, the possibility always exists of a cut or wound getting infected. Fortunately, most harmful bacteria found in your tank water will be gram-positive bacteria, and these can be kept at bay by a healthy fish's natural immune system. Gram negative bacterial infections are usually only seen on new wild caught fish, which hopefully are in QT before being added to your DT. Some gram-negative bacteria can even be spread to humans (like vibrio) so be mindful of that when working in the QT.
More info on bacterial infections here: https://www.reef2reef.com/ams/tank-terror-bacterial-infections-in-the-reef-tank.22/
So, can a DT really be disease-free then? Yes, but it is not easy. Knowing disease can only be introduced by a foreign source, you must do everything in your power to prevent that from happening:
#1 QT all new fish purchases, even your very first one: https://www.reef2reef.com/ams/how-to-quarantine.232/
#2 ISOLATE all new coral/invert purchases for 76 days in a fishless environment: https://www.reef2reef.com/ams/how-to-quarantine-coral-and-inverts.228/
#3 Aerosol transmission is a real thing! So, observe a 10 foot radius when housing any QT from your DT. It's also a good idea to do the same for separate QT systems.
#4 Be paranoid about cross contamination! Each tank should have its own dedicated equipment, water change hose, feeding apparatus, etc. Don't "share" anything between tanks without properly sterilizing it first.
#5 Make your own water! Cross contamination is just another reason to buy your own RODI, and not rely upon a LFS for freshly mixed SW.
However, the truth is no new tank comes preinstalled with ich. Cryptocaryon irritans aka "ich" is a ciliate protozoa that is either brought in on an infected fish or from water, rock, coral, etc. that was taken from an ich-infested environment. Let's break down all the different ways a disease can be introduced into your DT:- A fish carrying parasites or worms - the most common way. And just like germs on another person's hand, parasites/worms cannot always be seen on a fish.
- Parasite tomonts or "eggs" encysted on a coral or invert. Parasites actually reproduce "off fish" - tomonts will stick to almost any hard surface, whether it's an SPS frag or exoskeleton of a shrimp.
- Aerosol transmission by housing DT & QT too close together: https://www.reef2reef.com/threads/aerosol-transmission.190292/
- Cross contamination by sharing equipment, hoses, feeding apparatus, etc. between two tanks. Anything wet (including your hands) can transport free swimming parasites from one tank to another.
- Water. How many of you are still buying SW from a LFS? The same LFS that is notorious for harboring diseases in their system. Do you really think LFS employees think twice before sticking their hand in a tank and then putting that same hand in the SW vat?
All tanks start out disease-free, unless it is "inherited" by using rock, sand, water, equipment, etc. from an already infected system. It must be "introduced" in some way - just like pods won't magically appear in your tank without a source (rock, chaeto, bottle of pods).What about bacterial infections?
When you cut yourself, you apply a topical antiseptic or antibiotic to ward off the possibility of infection. Unfortunately, this isn't a very practical thing to do when a fish gets cut, stung or otherwise damaged. (Although there is a product called Bio-Bandage for just this purpose.) Unfortunately, the possibility always exists of a cut or wound getting infected. Fortunately, most harmful bacteria found in your tank water will be gram-positive bacteria, and these can be kept at bay by a healthy fish's natural immune system. Gram negative bacterial infections are usually only seen on new wild caught fish, which hopefully are in QT before being added to your DT.
Some gram-negative bacteria can even be spread to humans (like vibrio) so be mindful of that when working in the QT. More info on bacterial infections here: https://www.reef2reef.com/ams/tank-terror-bacterial-infections-in-the-reef-tank.22/
So, can a DT really be disease-free then? Yes, but it is not easy. Knowing disease can only be introduced by a foreign source, you must do everything in your power to prevent that from happening:
#1 QT all new fish purchases, even your very first one: https://www.reef2reef.com/ams/how-to-quarantine.232/
#2 ISOLATE all new coral/invert purchases for 76 days in a fishless environment: https://www.reef2reef.com/ams/how-to-quarantine-coral-and-inverts.228/
#3 Aerosol transmission is a real thing! So, observe a 10 foot radius when housing any QT from your DT. It's also a good idea to do the same for separate QT systems.
#4 Be paranoid about cross contamination! Each tank should have its own dedicated equipment, water change hose, feeding apparatus, etc. Don't "share" anything between tanks without properly sterilizing it first.
#5 Make your own water! Cross contamination is just another reason to buy your own RODI, and not rely upon a LFS for freshly mixed SW.
