Solve one missing rectangular dimension from a target volume and two known measurements.
Rectangular geometry can solve one missing internal dimension when you know the target litres and the other two dimensions. It is useful for concept planning and checking whether a stated capacity is geometrically plausible.
A target of 180 litres equals 180,000 cm³. With a 100 × 40 cm base, the missing water height is 45 cm.
Reverse geometry solves a size; it does not prove the resulting shape is suitable for a particular animal.
Swimming length, footprint, surface area, weight, glass design and species behaviour remain separate considerations. Treat this as geometry, not an aquarium-design approval.
If you know a target rectangular volume and two dimensions, the missing third dimension follows from simple geometry. This is useful when comparing cabinet footprints, sketching a custom sump or asking what height would be needed for a particular footprint.
The result says nothing about whether those proportions suit a particular animal. A tall narrow tank and a long shallow tank can have the same litres while offering very different swimming length, surface area and usable footprint. Research species needs separately.