I don't think any system is 100% fool-proof. When we talk about impacts to the environment, the land-based system is first of all going to be discharging effluent that will have nutrients in it. Unless those nutrients can be completely removed from the environment and it's not completely separate.... If you have a lot of nutrients coming out of a closed containment system and now it's going into the ground or going into the ocean, then in my view it's not 100% contained.
Also, it is possible for disease organisms to show up in closed containment systems. In fact, it is a bit of a conundrum, in that if you do get a problem it's very difficult to manage. There have been cases in the past in which closed containment systems have basically gone out of business because they didn't follow sufficient procedures to prevent the introduction of a pathogen. That being the case, the pathogen can get in and then obviously a pathogen can get out.
Having said that, I think that by and large a recirculating aquaculture system system would be a closed system, whereas an ocean-based system is not a closed system.