Assuming those dimensions are in inches your current tank is nearly 17 gallons and there's no rush to move to the bigger one (see Conversions and Aquarium Volume Calculator @ petfish.net). However it wouldn't hurt and 18x18x20 would be a great upgrade. As a bonus if move to a bigger size you can use the smaller tank to house new frogs
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I'd pend your money now on adding some real plants. Pothos (devils ivy) is an easy to care for vine. Some of the upright growing Peperomia obtusifolia would give some instant cover and perches low heights and will eventually fill upwards. There are hundreds of other options.