Ok, here goes:
I'm roughly assuming the biggest boxes i've seen are 18"x18"x3'. Some kind of rig suspended from the top of the tank would need to be fitted to sit the boxes on unles you want a couple of colums going all the way up from the bottom! First box would have some kind of mesh on the end for the intake and be your mechanical filter. A pipe would run from this to the second box which would be Bio and pump. Then pump returns water to the tank. When extension built, another pipe could run to a third box with more bio and the pump moved down also. This is very simplified and a rubbish drawing but i think you get the idea. Only problem is if your water changes drop the level rather than running an over flow and constant top up then the filter will need switching off when you drop the water level for the change.
If you use a pipe for the intake you can run it down to whatever level in the tank you'd like to extract your fishes waste from rather than just taking from near the surface.
As you can see, if you're having 2x3 windows at half hight then this will all be above them out of sight. Up to you whether you'd want it at the front or back of the tank. Also will provide a kind of surface cover which i'm sure the catfish wouldn't object to sitting/hiding under.

I'm roughly assuming the biggest boxes i've seen are 18"x18"x3'. Some kind of rig suspended from the top of the tank would need to be fitted to sit the boxes on unles you want a couple of colums going all the way up from the bottom! First box would have some kind of mesh on the end for the intake and be your mechanical filter. A pipe would run from this to the second box which would be Bio and pump. Then pump returns water to the tank. When extension built, another pipe could run to a third box with more bio and the pump moved down also. This is very simplified and a rubbish drawing but i think you get the idea. Only problem is if your water changes drop the level rather than running an over flow and constant top up then the filter will need switching off when you drop the water level for the change.
If you use a pipe for the intake you can run it down to whatever level in the tank you'd like to extract your fishes waste from rather than just taking from near the surface.
As you can see, if you're having 2x3 windows at half hight then this will all be above them out of sight. Up to you whether you'd want it at the front or back of the tank. Also will provide a kind of surface cover which i'm sure the catfish wouldn't object to sitting/hiding under.


Or go with the canisters on the outside too.
for the ideas, totally mint