I don't know that I've ever not had to deal with aiptasia in a tank. You will always get a frag with a tiny aiptasia that you miss and a week or two after putting it into your tank you'll be able to see it. There is really no avoiding them, quarantine helps a lot but ultimately unless you're willing to toss a frag over that aiptasia that pops up, you have to deal with it just preferably before it reaches your display tank. I've seen them on snail shells, on frags, and as you said do your best not to introduce any contamination via water. My best piece of advice is don't fear aiptasia just be immediately ready to deal with one while its still one. If you think its no rush and you can deal with it while its one or two, no stress, It'll become one hundred or more and way less fun to deal with. Berghia do a great job, but once you've let them spread it becomes a game of reasonable containment rather than eradication.