No really, 2 x 200Watt heaters in there will be fine.
A large tank with piranhas or Oscars look great with nice rockwork and some braches.
As said with a few good filters turning over the water well and with no sand or gravel your water will be kept clean. All you
Have to do after setting up that is to change 20-30% of the water every month ideally with RO water.