I realize the average aquarist might consider a HOB filter, with an airstone or 2, in a 4 ft tank, akin to be simulating a riverine system,
but I find that wimpy non-directional water movement and tank size so lacking, compared to even moderate flow rates in nature, as not feasible trying to force a group of Geophagus, Thorichthys, or Cribroheros (normally considered social cichlids)
or even Heros, as a group, not realistic, or adequate.
Minimal size, ¨slighty directional¨ riverine system might be, 5 HOB filters lined up on the back of 6 foot tank, and enhanced with a low powered wave maker located on one end, pushing directionally along the tanks entire length.
This may give some semblance of moderate riverine flow, rate for non-rheophillic geophagines, and some more small Heros species, or small riverine Central Americans, or those from riverine habitats west of the Andes, such as juvie Mesoheros, and Andinoacara.
I know I harp on this a lot…..
But for true rheophillic social species, to live even remotely, and socially, as they do in nature, a much more robust flow rate across the length of that same size tank may be required, just to get close, for true rheophiles used to living in faster flows, in massive water courses.