[ThRoewer]

Paul, I have a few questions:
- How many fish do you buy in a year?
- Where do you buy your fish?
- How do you select your fish?

I'm assuming you buy few fish, usually locally and select them carefully.

Back in Germany I didn't do much quarantine either - usually I set up new tanks when I planned to buy new fish (I ended up with more than 30 tanks + 22 larva and grow-out tanks).
At that time I selected my fish rather carefully and got livestock primarily from De Jong and a couple of local stores who's owners I worked with.
But even with careful selection I bought some fish that were sick - mostly rare clownfish (percula were super rare back then in Germany) where I knowingly decided to take the risk and treatment usually turned out to be required.
Back then I fed my fish a lot since most were broodstock and pretty much all were spawning like crazy. Since it were all fish only tanks I ran my salinities low at around 1.017. That alone helped quite a bit.
The tanks didn't look pretty with GHA, RHA and bryopsis everywhere, but the fish didn't care much about that.

Ich is usually only a problem for people who overstock their tanks with incompatible fish or have other systemic issues. And of course when buying fish that already come with a significant ich infection.

I've read up quite a bit on immunity and yes, fish can acquire various degrees of immunity to a wide multitude of parasites, including Amyloodinium and Monogeneans (skin & gill "flukes"). But it usually requires surviving an infection and then some recovery time to build up the defenses to that particular parasite. Though during an active and escalating infection with such fast replicating parasites fish have not the energy resources to build up immunity.

However, there are other things that can kill fish so fast that they have no time to build up immunity. Those are parasites that generally require treatment. Those things are the reason why I quarantine these days.

If you think otherwise then I dare you to get a few really sick fish with velvet or flukes and toss them into your tank. I don't think you will take that risk...

BTW: my main system has some low level ich and the fish seem to handle it fine. Flare-ups usually happen only when a new fish is added who disrupts the hacking order or due to other significant stressors. It usually goes away on it's own.