Lets see if I get the question, you are on mains power but need back up power for fish ponds, that part is understandable.
Now you have a large battery bank and two adequatly sized inverter chargers with a 6Kw load. You top up the batteries during mains outages by a 13kva gen set, and your power outages are frequent, and have no source of renewable energy.
Just quickly, when the gen set is running supply your 6Kw load and charge the batteries, changing to DC motors or generator does not make sence to me. Manage the battery discharge and start the genset when an acceptable discharge level is reached, when the mains returns use that to finish charging the batteries if necessary.
Any better answer would require more detailed information. There was another question recently about fish ponds running off a gen set.
allan down under