As another suggestion, how about submerging the exit of the drain line and planting the sump? If you prefer more stable CO2 ppm, put the main and the sump on opposite photoperiods. This sounds difficult to dose but in reality is easy with EI. FWIW I ditched my DIY wet/dry a long time ago and just use this method with plants as biofilters. Works well for me.

If the tank is drilled, Durso standpipe is the shiznit -- quiet, low surface agitation, allows high flow with minimal-no CO2 loss -- www.dursostandpipes.com

Hope this helps.