I got the citric acid powder from a farm supply store nearby. But I needed more (it's a great cleaner for all kinds of stuff, and very eco-friendly), so I ordered it from a local brew-it-yourself winemaking store. THey had little packets for like $5, but I ordered a kilogram (2.2lbs) for around $10. I used maybe half of the kg for the cat, so about $5.
IIRC, the powder was diluted in water using a "formula" of 100grams in 1 litre makes a 10% solution. Or 1gram in 100ml makes 1%. Or maybe you are supposed to count the water as part of the mix - so it's technically 1g of powder to 99g of water for a 1% solution. Sorry, I didn't get very far with high school science.
The Cypriot research (if you google "Reactivation of an Aged Commercial
Three-Way Catalyst by Oxalic and
Citric Acid Washing", one of the links gives you the article for free) also used OXALIC acid. They compared the two different results. I think they said oxalic was BETTER for certain contaminants. But that citric alone was almost as good, so I just used that. Again, the research was for diesels. They used a 0.1M solution, and I don't speak "chemist talk", so I don't remember how to convert that to %. Maybe someone smarter than me can explain.
I think I used around a 10% solution? I know I use a 5% solution to clean the toilets, trying to keep it pretty mild. So it must have been around 10-15%? Nuts, I wish I remembered the exact details. If you google citric acid in relation to de-scaling kettles or boilers, there is tons of advice. I think lemon juice is around 5%. Or figure out what 0.1Mol means.
Sorry, I should have written all this down. I'm the type that figures out the answer for the question at hand, and then promptly forgets most of the details. Plus it's been almost a year.
I didn't use any special water - except to say that I have multiple filters (carbon & coconut shell based, NOT RO) on the tap water, and our water is not very hard, and doesn't have lots of iron, etc. I suppose the best water would probably be distilled. I think RO is too "hungry" because it's been stripped of pretty much everything, making it unbalanced. And that's why you're not supposed to drink it. But this is another can of worms, and the Coca Cola company would probably disagree.
Sorry, I don't know what water softener cleaner is. Is it some kind of solution that you are supposed to use to periodically de-scale your water softener? We don't have one, so I'm just guessing.
Hope this helps.