First fight with ich unfortunately

[Koikid88]

After years of keeping koi, I've finally encountered Ich within 2 of my tropical tanks.
It is still in early development, I'm fairly confident I caught it early, and have begun to treat with salt and heat (Fingers crossed that's all I need). 1 tank is now up at 86, going to 88, while the other smaller tank is currently at 82, working up to 86.

So my question is;

Aside from water conditions, what causes ich? I've read up on it on many sites, from the life cycle to exactly what the parasite is, I just haven't narrowed it down to HOW it enters the tank.
Just say the water is absolutely perfect.
How would it be introduced? A foreign substance from the air, or do all fish have it before it develops into 'Ich'? If they all carry it naturally, what makes it develop?
I'm not looking for simple answers such as stress and water parameters or introduced fish are the cause.

Are there any scientists among us?? lol
Or any sites that explain this. (I've read articles on most of the more popular sites)

 

[Jesse18]

Feeder fish. not qt a fish And sticking it in your show tank

 

[Koikid88]

I don't feed with fish, blood worms and brine shrimp every few days with flake and pellets the other days

 

[Mumma.of.two]

It gets introduced into your tank by any fish you put in. It can lay dormant until the opportunity arises (weak fish from poor water quality, stress, etc). Once you have beaten/treated it successfully it should not return unless introduced again by a new fish.