This would be a very interesting project Paul. In the past I have only used this method with minor car body repairs. For the last 40 years I have been in the industry has a repairer and specialist spray painter.
I would be very interested in this concept. But I would like to know about the strengthening of the tanks! with holding so much water volume.
If for example we were to say a one meter square tank! How thick would the walls have to be? How much fiberglass matting would be used, and how many liters of resin.
The concept is good! It would be interesting to know the price.
I should be able to tell you tomorrow.
The construction of the tank would be, most likely, wood, with the fiberglass cloth laid in strips, in layers, across it. My largest aquarium was about 65 US gallons, when I was a boy. I never had an issue with it holding water. I just have to make proper calculations. Otherwise, I will end up with a whole lot of water on the ground.
Seriously, I am going to put the sump completely in the ground. The fish tank will be, at least, mostly in the ground. So, the walls of either tank, honestly, do not have to be too terribly strong, due to being supported by the dirt / clay in the ground.
If either were going to be freestanding, the walls of the tanks would have to be considerably stronger.