Hey, I actually remember this one. After getting confused just where I dropped off on the original airing, I was surprised to hit one in the back half of season 5 that I recognised. I know I saw Equinox, but I've not recognised one for so long.
Nice guest star, and interesting plot with the double crosses and it all being a big massive ploy to get Seven on board their ship. The Think Tank themselves were pretty awesome, with such a varied range of aliens, it's something Star Trek doesn't do enough of. And I couldn't help but wonder if the guy in the fish tank was somehow connected to a certain probe in a film.