Many have performed pressurized fermentation or cold crashed in our conicals. One question we get a lot is that the pressure lowers over time, suggesting a leak. 

In fact there a few reasons why the pressure in your conical may be dropping

  1. Temperature drop. Pressure is directly proportional to temperature, if one lowers, so does the other!
  2. Fermentation is slowing down or complete. The residual carbon dioxide in the tank is then re-absorbed by the beer, much like if you were to carb the beer.
  3. You took a sample or purged yeast or trub.

If you want to hold a fixed or minimum pressure on your conical, it's recommended you keep your CO2 tank attached to the conical.