I'm no optics expert, as you will see, but i would think refraction would be a big problem whether you keep perpendicular to the tank or not.
I attach a sketch of a possible solution, which won't completely solve the problem as the sucrose solution presumably has a different refractive index to glass (i don't know how much different - it might be negligible?), but should at least counteract the effect of refraction when passing through the air to 'non air' interface.
A concave-plano lens placed on the tank glass surface, and moved with the camera, where the radius of the concave curve is equal to the distance from the lens to the camera's lens nodal point, which is perfectly centered at that point, should ensure that all rays passing through the concave plano lens toward the camera's standard lens exit more or less normal to the concave curve and therefore do not appreciably refract.
This may be practically very unlikely if not impossible but someone might like to have a go!

*edit - but now after looking at my own sketch i can see that in the example where the rays refract and there is no special lens, it is just the same as standing further back and shooting with a longer focal length, so its probably fine so long as you keep perpendicular! do you even have to keep perpendicular?