It's perfectly normal to have some microfauna in an aquarium and they can come in from poor husbandry or in plants. Sometimes they just show up. They can look tiny, like a layer of dust on the inside of the tank until you look close enough and see them swimming or they can be larger and more noticeable. Here's a good site to look at when it comes to microfauna:
It's mostly about worm-type microfauna but they have sections about other types as well.
One thing I've noticed is that chlorine doesn't seem to touch many species of microfauna... I QT my plants in tap water (without conditioner) and still end up with things in my tank. Most of them are harmless - especially the teeny tiny ones and they can actually be beneficial in multiple ways (whether as mini janitors or live fish food).
Upping your water changes temporarily will certainly reduce numbers in the water column. I had an infestation in Kaze's first tank that seemed absolutely impossible to beat but with an extra 30-40% water change a week it helped clear most of them. If you're tank is cycling the last thing you want to do is empty it and blast it, but if it's one you do 100% water changes on anyway then it doesn't really matter. You shouldn't need AQ salt or any treatments for the tank as these things are most likely just microscopic lifeforms but you must do what gives you peace of mind