Not all aquaponics requires a greenhouse. If you live in a sub tropical or tropical climate a greenhouse has some challenges that might be easier to deal with by skipping the greenhouse in some cases.
There is far more to an effective greenhouse environment than simply a plastic cover. In a hot climate a simple plastic cover is an effective way to cook your plants and trap the pests in with them. In a really cold climate a simple plastic cover isn’t enough to keep things flowing through the cold season. Decide carefully if that simple plastic cover is going to be appropriate before you depend on it to make your growing easier. I’ve found for effective greenhouse growing some automation and climate control (even if it is just thermostats that can open/close vents and turn on/off fans) are rather important. Few people have the time/patience to always be on hand to open/close and turn on/off ventilation in a simple greenhouse when it is necessary.
I happen to live in an inland, humid, subtropical climate. We do get freezes and frosts occasionally but most of the year (sometimes within 24 hours of one of those frosts/freezes) it is too hot for a sealed up greenhouse. My first two Aquaponic winters I was trying to run under a simple plastic cover type greenhouse and I found that when it was warm and sunny, the greenhouse tended to get too hot and the cool weather crops suffered but when it got cold at night, it was cold enough that some of the warm weather crops didn’t really like it much either. So there I was during Florida’s best growing season with a bunch of plants that were suffering from spider mites and aphids inside a greenhouse because I was trying to recover some heat to keep tilapia alive through winter. My by third winter I was told I had to take the “greenhouse” down by code enforcement and that actually turned out to be a good thing. That third winter I lost all the tropicals in the system and didn’t try to grow tomatoes through winter but the cool weather crops did wonderful and I had far fewer pests problems in the aquaponic plants year round after that because I had better air flow and beneficial predators to help with pest control.
From now on in my book, greenhouses are for those willing to install something that can support some automated vests, fans and other heating/cooling devices. My schedule and temperament are not appropriate to monitoring greenhouse conditions 24/7 to deal with opening/closing things up and turning on/off fans and the like manually.