There are 3 or 4 manufacturers or Juwel length tubes, Juwel, Arcadia, Geiselman and iQuatics.
The reason they are custom lengths is due to the limited tank volumes you would get using industry standard length tubes, especially in smaller volumes under 300-400 litre. All to do with ratio of width to "pleasing" depth, for industry standard lengths give nothing in 100 litre to 300, litre range. Juwel have, 80l, 125l, 180l, 240l, 260l, 350l. These volumes are possible using standard tubes but tanks, in the above volume range, would look "unpleasingly" shallow. I did post about this, years ago after getting an email (2000' odd) from Juwel why they made their own length tubes.