The new system is going to be a simple CHIFT PIST* with fewer points to go wrong compared to my current system.
The problem with adding a window is that it's another point that could fail. The side of the IBC fishtank is flexible, and the glass window is rigid, and that never ends well.
So I thought I'd run it through the invention engine and see if there was a safer way.
It came up with something it likes to call the "No Holes Fishtank Window".
Which is nice.
So, it turns out you could probably force an aquarium partially under water from the surface to make an interesting inside-out aquarium.
It's a little reminiscent of Wonco's house, "Outside of the asylum".
A better viewing angle would be made from a wedge shaped aquarium made from two squares of glass, and two triangles for sides. One square sits flat against the side of the fish tank (the side you would be leaning against when viewing), and the other sits at an angle of 45 degrees, so that it meets the other square 40cm underwater.
The entire device is mounted on a bracket that keeps it underwater. The bracket is an inverted U shaped bar of stainless steel (stainless steel, so the fishies don't get all dead). One bit of the U is siliconed onto the side of the window, the other end has a locknut to attach it to the frame of your IBC.
It could be easily removed to clean, or to simply add to land fill.
It will probably work, and might look a bit like this...
I might even make one for my boat.
120 Things in 20 years is still alive!
*Constant height in fish tank, pump in sump tank