Stacking against the glass will limit flow around the rocks. Also detritus will build up significantly back there, making cleaning behind impossible. This will mean nitrate build up to some degree. An abundance of flow will help but it will not eliminate detritus buildup.
29g Mixed Reef, 150w 14k Phoenix MH w/ Moonlights, Reef Octopus BH1000 HOB Skimmer, Ecotech Vortech MP10es, ReefKeeper Lite (Plus), PH Probe, Temp Probe, ATO, Carbon/GFO Reactor.