Please read whole post before jumping on the "Oh noez don't use distilled water, it's horrible!" bandwagon...
I have done research on the net regarding using distilled water and I'm even more confused now.
I have found opinions ranging from "distilled is fine, I use it" all the way to "omg the fish is going to die a horrible death if you do" type of answers for this water. These opinions seem to come from average people all the way up to some type of aquarium site.
I've had my betta in distilled for at least 2 months now and first let me say this: he's fine as far as appearance, eating, activity, etc. He's not acting lethargic or breathing heavy. His appearance is great, no ragged fins or anything of the sort...etc., etc.
I've read that plants nor the fish can survive in this water, and either I have "superman" for a fish with 2 types of "superman" plants (that have to be trimmed every 2 weeks) or bettas are much hardier than what people think.
My tap is way too hard leaving a nasty residue on anything it touches and I'd rather add to the distilled if possible instead of being a scientist when it comes to my horrible tap water that is going to make my tank look not so pretty. I tried a slow swap to spring water and it caused a major spike in acidity among other things...pH was the most alarming for me though as the spike was off the chart (literally) when I did that so I've switch back slowly and it's close to 7 again.
It seems distilled is missing possible minerals the fish needs. Is there a way to put those back into the water without causing a major problem? What exact minerals are missing?
I've also read that it's a matter of simply adding aquarium salt to the water. How true is this?