There is lower O2 in water that comes from the RO/DI unit. Once in the tank I had fluctuation from plants but, with good surface agitation there's no problem.
But some algae in larger quantity can have an impact during night time. I never tested under 6 in the tank with somewhat a turnover that is much higher than normal, I think It runs around 15 to 20 X per hour, it's a 150 GPH on a 5 gallons, but it's restricted by the filter media. The plants disperse that very well.
Without the air powered filter in addition. O2 was lowering during the night, but never to be a concern. But still I prefer not seeing it go down at all. The amount of contact with surface area they create is under evaluated for sure.
