PlanetsCentral

In turn 18 the cactus score tells me I lost one. Indeed the fed conquered me a planet with a ship in ship-planet battle. In the same turn, however, I took a planet from the fed by unloading some colonists. Shouldn't the score have said I was at 0 for losses/gains

Only if the planet had a cactus. Otherwise, capturing it has no effect on the cactus score.

Basically, the gist of this setup is: you benefit if you build cactuses on planets that you intend to keep for 15+ turns. If the planet is in a battle zone, building a cactus is risky.

--Stefan