I have gotten one single 20G long with three sponge filters, and a massive overgrowth of moneywort to support two Convict cichlids. I know that this is overstocked, and yet I have never had the nitrate reading go above 25 on my API test. Please note, that I DO still do water changes roughly 50% every other week. They have non-stop bred like (well convict cichlids).
I assume that I am replacing enough of the hormones built up in the water that would impede breeding with this grossly nearly non existent W/C schedule.
Note 2: I am in the top 5% largest average water change percent fish keepers here on MFK, and understand that I am way overkill at 70%+ W/C's per week on the average with all of my other tanks.
You would not believe the "limited" space in that 20G long tank with the 3 internal sponge filters and Moneywort choking the water column, and growing just as heavy as much as 18" above and down the outsides of the tank.
I see no way possible of achieving what I have in the tiny 20G long tank with any tank over 40 gallons.