i do not have a coldwater setup but eventhough i have a freshwater setup which might benefit from RO water, i do not use it because of the water wasted during filtering.
just my opinion.
RO water is mostly unnecessary for fresh water. To use in FW setups you need to replace the essential minerals and salts (usually comes in a bottle) or mix with tap water.
If you have excessively high nitrates then you can use a mix of RO and tap water to save needing to put the minerals etc in.