Distilled Water

Basically the same.

You can use RO water in lieu of distilled.

ok brilliant, I'll give that a whirl then!

I had the same question. But also does the temp. of the water matter? Because my RO unit gives cold water since it's meant for drinking.

Hot water will allow more chemical to be dissolved until the water reaches its saturation limit. Hot water has a higher saturation limit then cold water. But as soon as the water cools to room temperature the water will be super-saturated with chemical and some will drop out of solution and just sit on the bottom in a pile.

Might as well mix at room temperature, or just don't add so much chemical that you put more chemical in then cold water can hold.

In case you were wondering, the saturation level is different for each and every chemical so I can't give you an exact idea how much to add before reaching this limit. You will know when the chemical stops dissolving and sits on the bottom in crystal form.

Alternatively, you could just use the fertilator application on this website (top left next to plant finder) and add dry chemical to the tank itself without pre-mixing. I do this, and its much easier in my opinion.