You can use tap water, some tap water is better than others. You should be able to go online and find information from your council/government body on how your tap water is treated and what is in it after treatment. Most tap water contains most micro minerals such as iron calcium magnesium zinc ect. So its often a good choice depending on where you are in the world and how it is treated. I believe the reason to leave it 24hours is to let the chlorine evaporate. Some say this is not needed but I suspect that's because they don't have heavily chlorinated water supplies.
As for cleaning your basement walls you can use a fungicidal wash first, wipe it all down, you can then spray with h2o2 but use it fast and strong. I would recommend sealing the walls with a cheap water based paint after that but at least a week before your plants are in the tent. That way if any mould spores return through plaster, brick ect you can see it very soon and sterilise it straight away. If your walls are crumbling then you can spray with PVA mixed to a ratio of around 5 part water to 1 part pva. You may not need to do all this but if its a damp basement it should help.
No idea why people think Canna is expensive, its cheap when you compare what you get from it. Plus the quoted price is for both parts, A+B.
Good luck