The protomont (= ripe trophont that has left the fish) may not be free "swimming", but it is free and motile to move around.
The protomont is in this regard just a fat theront and as such unprotected and vulnerable to chemicals (cooper, CP,...) and environmental conditions (hyposalinity) before it can settle down to encysts and become a tomont.
The protomont is also the most likely stage to fall victim to predators as it is relatively large and quite nutritious. Even fish may reportedly eat it if they can get a hold of it.
The stages that are generally protected are trophont (feeding stage - shielded by the fish's skin and ) and the tomont (encysted division stage - protected by the outer layer of the cyst)
The feeding trophont may be affected by chemical agents if they reach it via the bloodsream of the fish it feeds on. CP may work in this regard.
The fish's own immune system may in some cases also produce agents capable of killing the intruding theronts before they can settle in and become trophonts. Simillar immune responses may also be capable of limiting the theronts feeding.
The tomont (cyst) is only affected by extreme environmental conditions (heat, drying) or harsh chemicals (bleach, lye, acid) that are able to penetrate or dissolve its protective layer. Anything that is capable of killing the cyst stage will generally also kill everything else in a tank (at least everything you want to keep alive).