the water probably won't do much. you need actual bacteria, so filter media, sponges, etc where they live.
before you get another ray, i would suggest finding that 180g you're looking for, and getting proper filtration for it. canister filters, a sump.
your current setup with the 2 tom dive filters isn't enough. they are rated each for 29-40g of filtration. not nearly good enough. don't mess around with filtration when it comes to rays. being cheap about it will ensure future buys die as well.
if you get another motoro, you'll have to upgrade the 6x2x2 tank anyways, alot of motoros reach 20 inches in disc, if not larger. would you be comfortable in a room that is slightly wider than your shoulders?
hold off on buying the next ray until you've figured out these other problems first.