yea you can use ro water on a fw tank. i do on all of mine. you just will need to add back some trace minerals
i think it is. it does cost though so lol
I use RODI water with my 40 gal fresh. I just use 1/2 tsp of Kent RO Right in a 5 gallon batch
Used to have some stringy brown algae and within 4 PWC's saw a world of difference. Like you, I thought about it and said, what the ****. I already own this unit might as well use it for both tanks.