Wow, that's quite the undertaking! A 700L and three 300L tanks – impressive! Avoiding CO2 makes things a bit trickier, but definitely doable. You know, it reminds me of that Granny game, where you're stuck figuring things out in a strange place with limited clues. I'd suggest looking into heavily planted tanks with good surface agitation for natural oxygen exchange. Start researching some O2-based systems specifically for larger tanks. Good luck keeping it a secret!