What's The Best Way To Use Parasites?
A: Use your parasites or other "reclaimers" as support ships mixed in with your main group of units. Let your main group deal most of the damage while your parasite/leach/nanoswarm units need only one hit to tag the enemy for reclamation.
Parasites are part of a group of ships with the "reclaimer ability" that also include; Leach Starships, Mercenary Parasites, Neinzul Youngling Nanoswarms, and Botnet Golems. All reclaimer class units only need one hit on a ship that is not "immune to reclamation" to tag that ship for reclamation when it dies. Reclaimed ships will be under player control and start with 50% or less health based on the damage done to them by ships with the reclaimer ability. The Botnet Golem is the exception, in that it reclaims units at full health and its' reclaimed units are zombie bots. See Botnet Golem for more info.
The other major consideration when using "reclaimers" is energy; each ship you reclaim costs you energy, even though it is otherwise free. So before you send a lot of parasites, leaches, or nanoswarms into battle, it's a good idea to make sure you have an excess balance of energy (since some of your services shut off if your energy goes negative, such as tachyon beams, force fields, tractor beams, etc).
Reclamation tags do eventually expire if the unit survives, and are instantly removed when units traverse wormholes.
NOTE: It is possible that reclaimed ships will have only a few points of health when you get control of them if most of the damage was done by your other units. Consider building a "rally point" in a safe area and turn on the "reclaimed ships rally to the rally point" toggle in the "Galaxy" tab of the "CTRLS" button. Reclaimed ships will then automatically head for the rally point as soon as they are reclaimed. Add a Mobile Repair Station within repair range of the rally point to start repairing captured ships quickly. Additionally, Mobile Repair Stations in a system where reclaimed ships are being taken will act as an automatic rally point.