I kept my mated pair of D. Tinctorious "Azureus" in a twenty gallon long for close to two years and they bred like crazy. I ended up moving them to a twenty gallon tall because the 20 long just got too overgrown and yucky. I designed my 20 long so there was driftwood up against the back and allowed them to climb a little.

I believe the length was more important to them than height.