That is Interesting Dave!
Well I plan to setup a 29g after I move. I just broke it down for nothing, even Wisteria, was growing in the tank. Probably because substrate was river sand from local place. I always have algae problems in it. I think I need to increase the circulation. For I have now set up a 20g long with the same filter, an Eheim 2213, and the same light strip, an Odyssey 24w. and the black algae on the plants has disappeared.
Due to tight funds I will probably put a powerhead in with the Eheim 2213. If I have a powerhead on 1 end and the Eheim outflow on the other side will the out flows be pushing against each other?