Noahs ark6
Fish Herder
i would say get an external, If you do make sure you get one thats rated for a bigger aquarium then you have E.G a Fluval 205having the same problem, ran with the sand for last 3 months and just not happy with the look,thiniking of going back to gravel.
However reading this i wonder if its as simple as the flow. I have a juwel 125lts tank with internal filter,which is a 600l/h power head. So my question to you guys should i use an external filter or a more powerfull head?