Very lucky mark! When I thought I had them beat, I did my first water change prematurely... The next day it looked like I had done nothing, the explosion in dinos was significantly worse than it had ever been. Really sucked.. Months of work gone overnight, I almost wanted to cry lol.
I'm guessing it was a bunch of cysts that had opened due to favorable conditions. They'll form a cyst and wait you out and hatch when it feels right.
So if you beat them, keep an eye out, maybe a few weeks to a few months later, some will start coming back out of their protective cysts, and it's back to battle IMMEDIATELY or they will re-establish.
After finally getting rid of mine completely, they came back three times. The first time I thought it's just a few -- huge mistake. The second time and third time I went to war immediately, took zero chances. Saw one growing on a coral? Removed coral, cut off the growth and tossed it, dipped the coral in FRESH WATER, and put it back... Coral wasn't thrilled but it lived, dinos didn't.