What Are The Perfect Conditions to Make or Keep Fish Healthy?

A Perfect Environment?

For fish to recover from diseases and disorders they should experience minimal crowding, clean, pH-balanced well aerated water, good food and hiding places. Subtle differences exist in pH requirements among fish but the regular hobbyist will do fine with 'neutral'. 

To provide the ULTIMATE in hospital / treatment tank water, sponge filtration is THE BEST. 
The filter at right fits in a 20 gallon tank but it's made for 120 gallons, and would be many times the needed filtration but you can't have "too much" but here's a link to other sizes with the correct pore size. Aquaneat has the right pore size. 

Sponge filtration is great because it won't suck up the fish, not even baby fish. And it colonizes super fast and it's easy to clean. It runs with an aerator, not electricity. So the "power" is OUTSIDE the tank. Better still, that makes the tank essentially 'silent' but very well aerated. Because the aerators are sometimes noisy, I hide the aerator and run a long supply line to the tank. At home, I run an Aquascape Pro Air 60 with a supply line from outside the house. 
On a small tank use THIS ONE. 
At right, an air pump good for up to 500 gallons. I own 4 of these 45L units and they run for literally: YEARS

You have to get air from the air pump gang valve to the filter. I really, really like silicon airline tubing and if you haven't used it, you're in for a treat because it has no memory and so when it unrolls is straightens out, and drapes, instead of trying to roll back up. 
At right, is a BIG FAT roll of silicone tubing. You could be two or three PACKS of tubing, or this practically "lifetime supply" roll for about the same money. 

78 degrees is good for any fish recovering from an illness and accelerates clearance of white spot. 78 degrees also bolsters the fish immune response to fungal infections. 
Getting 78 degrees is easy with tank heaters, and my favorite is titanium, with an easy to read LED display and controls ON THE CORD and not on the unit. See the unit at right. It's all that. Also, it's 400W which means in a tank of 20-30 gallons it can hit the mark with minimal "on" time. The heater you see at right is too big for a ten gallon tank. 
You could use one of THESE for a 10 gallon. 

The system you put the fish into has to have biologically active media - beneficial germs that digest nitrogen. It's easy to get these from a healthy system already at equillibrium. Even the pet shop may spare you some media-squeezings.  At right, click the button to read about how to "bio-seed" a new system. 

There are tons of ways to bring and hold a pH to "neutral" which is the "important" thing. My preference, among LOTS of good ways to peg the pH, especially in a hospital or quarantine, is SeaChem's Neutral Regulator. 
They make a "liquid" version  but what I have noticed about liquid pH regulators is LACK OF ECONOMY.
Beginners could apply a dose to their tanks once a week and avoid pH issues forever.

So you set up a quarantine or hospital facility and you're seeing cloudiness in the water, and some cloudiness is okay if it's mineral haze but if it's bacterial you need to knock it down. You can use a water clarifier which works AMAZINGLY with a sponge filter as above. In tanks, the one at right (Accu-Clear) is the best. 
There's a POND version of clarifier that works well with a little better economy. 

A substrate in a fish tank or hospital tank also colonizes with beneficial bacteria and is a welcome sight to sick fish. I left gravel behind in my tanks about 6 years ago and never looked back. I use black sand. I've used black Fluorite. It's good too. The thing about black sand is that it intensifies colors in fish and it doesn't coagulate a bunch of fish poop. I get little snowdrifts of fish poop SOMETIMES but normally the poop gets swept up into my sponge filters.