I dont know what the filters are like in those tanks personally but dont know of any good filters in those sorts of tanks..
As for stocking you could stick a whole heap in that sized tank I have a 540 litre 5ft and would easily stick say 10 electric blues, 10 yellows and some peacocks, an even nicer display would be all males if he wanted to go that way if its a display tank breeding probably isn't such a big deal..