Our aquarium stands will advertise for a specific tank size. The Flipper, for example, can support either the standard 10- or 20-gallon tank, depending on its orientation. While these stands are designed with a specific tank footprint and weight in mind, it is not necessary that the aquarium match the shape or parameter of the stand. It is okay to use with an in-between sized tank as long as:
1. The tank does not exceed the weight capacity. If your tank is 13 or 15-gallons, or taller than it is wide for example, you would not be able to place it on a surface designated for 10-gallons as it would likely surpass the maximum weight capacity.
2. The tank does not hang over the edge of the stand in any way. "20-gallon long" tanks generally will not fit on a surface designated for a 20-gallon tank, even if it is within the maximum weight capacity. Always check that the dimensions of your aquarium do not exceed those of the stand.