This is a question that comes up more often, especially with so many un-moderated forums, which allow for the passing of incorrect information as fact.

Part of the reason for this web site (Fish Beginner) was to utilize researched, experienced, fact based information and make it more simplified.

The problem is, with so many aquarium keeping forums and customer reviews with little science and long term experience behind them, factual information is getting more and more diluted. The popularity of web sites such as Amazon, Pet Solutions, Dr. Fosters (Live Aquaria) and many more, which sell products that are often well marketed hyped with good mark ups, but often with little science behind them, further clouds the issue of "What Makes for Healthy Fish in an Aquarium".

Much of this information is courtesy of the web article:

A Healthy Aquarium; Disease Prevention

I will attempt to use a question and answer format along with a using a fictitious aquarium keeper as an analogy based on readings from Amazon Reviews, forums and similar to address these question and then provide scientifically/factually correct answers. 

What factors can lower fish resistance to disease?

The overall idea of a healthy aquarium comes down to keeping the best known possible environment for the aquarium fish. This is taking simple and logical steps to ensure a life, which is as long as possible. The physical health of an aquarium (or even human health) can be explained by a few concepts, which includes the important idea of Redox. 

Redox is a balance referring to the break down of the body or even objects, as well (Rust on a metal bumper is another idea of Redox due to oxidation).Redox is broken into two parts.

There are Oxidizers, which harms fish long-term health and trend to the positive mV side of the balance. There are Reducers, which improve health (immunity) and trend to the negative mV side of the balance. The goal of a good aquarium redox is to have a balance of a reasonable amount of Oxidizer (acid of fish breakdown is a oxidizer) and Reducers. 

There's natural processes, which affect redox (aquarium long-term health) in a positive and negative way. There are methods of aquarist husbandry (the way we care for our tanks), which can affect the aquarium tank in a positive or negative way.

A quote from an experienced professional:

"Redox, also known as Redox Potential, oxidation potential, & ORP (oxidation reduction potential) describes the ability for the loss of an electron by a molecule, atom or ion to the gain of an electron by another molecule, atom or ion.

Without this ability to gain electrons, many minerals cannot be absorbed and properly assimilated, especially in times of stress.

So it is very important to keep a healthy Redox Balance via proper dissolved oxygen levels, proper positively charged mineral levels (such as Calcium and Magnesium), and even level 1 or higher UV Sterilization."

A healthy Redox Balance can generally be measured via rH [relative hydrogen] using a Redox Meter and accurate pH test. A rH of 23-26 is generally considered healthy.

Reference: Aquarium Redox; by Carl Strohmeyer

More on the subject: Redox in Aquariums

Here are other ways to improve aquarium health. 

Know which conditions are Oxidizers and Reducers. This will greatly improve long-term health of the fish.

Regular water changes, but not overly large volumes of water. In otherwise well maintained aquariums, generally 25% should be more than adequate. With larger water changes, parameters can have larger swings, which would affect fish more. Smaller changes eliminate this issue.

Good filtration/Circulation keeps water from becoming toxic and also provides oxygen to the fish.

More than one form of filtration is suggested for Ammonia, Nitrite, and Nitrate control. This is done with aerobic and anaerobic bacteria, the first (aerobic) to control the Ammonia and Nitrites, the second (anaerobic) for the Nitrate control.

There is also chemical filtration for cleaniness of the aquarium, although some can be oxidizers and thus MUST be countered with products that also reduce such as AAP Wonder Shells.

Our Recommended Product Resource:

#AAP Wonder Shells for Aquarium Water Reduction

True UV Sterilization, which is using a certain light source to kill bacteria and viruses and also has scientifically been proven to benefit Redox (fish immunity).

First, the UVC positively electrically charges the water, which is the MAJOR difference between nature water and aquarium water (Aquarium water loose electrical charge, by being recycled in the tank). UVC allows for more reducing to happen in the aquarium.

A quote from a experienced professional:

"Part of the reason for true UV Sterilization is that the UVC irradiation which is contained in a a high dwell time UV Sterilizer will break down oxidizers in the water column [via Redox Balance] that would otherwise lower a fish’ immunity, this aspect is often missed as many only focus on the clarification/algaecidal properties of UV Sterilizers, which is why many persons now purchase low price UV clarifiers from discounters not realizing that there is a difference between what is marketed as a UV Sterilizer by in reality is only a UV clarifier".

Reference: Aquarium/Pond UV Sterilization; Sterilizer Use

Water parameters do need to be monitored. Ammonia should be close to zero as should Nitrites. There can be a small amount of Nitrates, up to about 40 ppm, with no affects. pH should be stable, which is more important than trying to find a certain pH for fish species. This is done with a KH of 50ppm or higher, which stables the pH. GH should be maintained anywhere from 100 to 500 ppm.

Important mineral Cations (part of GH) are HIGHLY recommended for proper Redox balance (aka fish health/immunity and lowering oxidative stress). Proper minerals improve very important body function of the fish.

This is essential for long-term health and is often MISSED by fish keepers.

A quote from experienced professionals:

"More importantly as to respiration, the surface epithelia of gills and body surfaces are protected from direct interaction with the environment by mucous and intercellular junctions. Fish mucous has been postulated to have calcium binding properties.

Mucous is a glycoprotein and could serve as a calcium chelating agent retarding ion loss from epithelial cells as a charged surface coat or barrier and thus is dependent on calcium for normal function..." 

This video helps demonstrate how mineral Cations remove oxidizers (including chlorine):

References:

Aquarium Chemistry; Complete

Osmoregulation in Fish 

In the wild, fish are presented with all different lighting types. Other than just if a fish is used to light or not, there is lighting types, which have been proven to improve health. This is also true for human health as well. Light with a 6500K rating is recommended and while it doesn't affect Redox, it does affect health.

Reference: Aquarium Lighting

Proper feeding of both the amount and type of food being feed is part of the equation for health, as this can have an affect on bodily health and tank water toxicity. Over feeding can harm health and also add to the oxidizers present in the aquarium.

What is in the food being fed is also important. Watch out for fish food with fillers. These fillers do not provide the necessary "energies" a fish needs for optimal health (including breeding).

Also look for foods with whole ingredients to provide vitamins & minerals, as vitamins naturally in the food is the only real way to get these benefits to the fish. Using all natural ingredients that do not need supplementation as well as slow dehydration processing over the high heat processing is key to an optimum fish food.

Another key point, whole fish meal is vastly superior as a source of proteins, minerals and vitamins over just plain fish meal [especially whole menhaden fish meal].

Unfortunately most common fish foods such as Tetra, New Life Spectrum and more are not up to these standards, despite their popularity. Better would be the ultra premium all natural AAP Custom Fish Food

Spirulina has been proven to improve immunity of a fish, so it's important to feed stand alone, or as an ingredient in another food.

Sometimes simply getting good nutrition into a fish is an issue with finicky eaters, weak, or stressed fish. For this there is no better prepared for than AAP/Gamma NutraShots developed by industry leaders in the UK

References:

Fish Nutrition, Complete

Health Benefits of Spirulina Algae

Fish Nutrition 101

A few recommended fish foods:

#AAP Spirulina 20 Fish Food

#Clay Neighbor's AAP Custom Fish Foods

#AAP/Gamma NutraShots Premium Fish Food for Finicky Eaters

Thinking about bio-load, or how many fish and foods are being used will help with health. Build tank communities wisely and with more fish make sure there is adequate filtration, otherwise bio-load needs to be decreased.

To much care is possible. Always having your hands in the aquarium adjusting filters or moving decor can have an affect on fish stress and water quality. Having your hands in the aquarium more, while the aquarium is being dialed in, is one tank. Always dosing the aquarium with conditioners (esp. oxidizers) and overly worrying about tank parameters and chasing them (esp. pH) is not ok. Treatment of fish should start at the first sign of disease, but no real conditioner is to be used all the time. Even natural treatments/conditioners can have a harmful affect long term. 

Knowing the activity of the fish in an aquarium is important, so if there is any changes in behavior, it will quickly be spotted. Clearly, if a fish is less active than normal this could be a sign of disease on-set. Fish sitting lower in the tank or resting on decor. more than normal this could also be a sign.

On the flip side of how fish could be acting, if fish are kept in a less than optimum conditions the fish will adapt to the living conditions, but this does not mean it's living the best quality of life it could be. Often times, when proper aquarium maintenance routine are started after a fish has lived in a less poor environment, fish activity will increase. This activity can look like increased aggression (or stress) of a fish, which in reality, the fish is over active, because of the improved conditions. This increased activity will likely be short lived until the new conditions are adjusted too. This could also be the new behavior of the fish. More so, with aggressive type fish. 

An example is a person who claims that their fish are doing just fine with few diseases and good growth and good color, even though this same person is using methods of fish keeping that maintains high levels of oxidation that in reality slowly degrade the fish' osmoregulation, disease resistance, or internal organs.

This is the same as someone pouring a herbal aquarium conditioner such as Melafix or Herbtana in their tank on a regular basis thinking it was improving the health of an fish, because it a natural medication. Little does the aquarium keeper know the medication in the conditioner is a water oxidizer, which actually harms the fish with the long term use.

A good example of this poor logic by applying it to ourselves would be a real life example of a couple of friends:

In once case I know of a person in his 50s that cooks from scratch, eats a very healthy diet that includes olive oils, fish, etc. yet he has had a very difficult year health wise. However I know of another similar aged person who eats terrible, get little exercise, smokes, and is over weight, YET he is doing very well. In other words this is an anomaly as we know statistically the person that maintains a healty lifestyle will generally have less incidence of disease and live longer!!

So applying the logic of the fish keeper who employs methods such as regular use of Microbe-Lift's Herbtana that slowly oxidize the fish and then make the the anecdotal statement that their fish are doing just fine if not better is akin to stating that smoking, eating a poor diet, etc. as in the human example is a healthy way of living.

OBVIOUSLY THIS IS LUDICROUS!!! 

Few Videos of Interest

Suggested Resources:

#Columnaris in Fish, Bettas; What is Correct?

#Atison's Betta Spa

#Two Little Fishies Aquarium Products; Including NPX Bioplasitcs