Yeah some cichlids will cross as their species are no that far removed but Oscars and Severums are two very different species so I dont think its breeding behavior - If they are quite happy to co exist than that is a very good thing for you as in a 72 gallon tank it could go hideously wrong, I notice a pic of a con in your picture what other cichlids are in with them? Im sure you know about the guides on oscars ie. 75g for 1 150g for 1 + tankmates etc. but just be careful with him, how old are they? I mean dont get me wrong it could work out fine but the guides are there because it has gone wrong so many times for other people.
As for the cleaning and digging thats what cichlids do all day long haha. My female nic digs none stop all day often with my gold sev watching her, her latest sport is building a pile in the corner then flapping her fins over the top of it till it goes down...... then the construction starts again....
Wills
