If you use 5g a day... that's pretty big system you have! And it certainly would make sense to setup something fancier than a manually filled bucket. Plus you need a bigger bucket anyway. I'd say at least 5days worth size. Then you can hook up RO/DI to fill it based on level in the bucket and drop regular ATO in there.
Or if you simply dumping fresh water into sump you can skip the bucket altogether (I pump my water to kalk reactor first, so I like having a bucket in between ).