If you have sand in your tank, the detritus or dead spots where waste build up are a good indicator that you need more flow. So first I’d try messing with the direction of the return pump in conjunction with the wavemaker and see if you can get full coverage of the sand bed, if not then I would get a powerhead. I have a biocube 16 a similar system to yours and I upgraded the return pump from 185 to 251 gallons per hour. Make sure your return pump is giving you sufficient flow, it’s basically the heart of the aquarium.