Tap water is 0.5 EC, and 8.1 - shall I get an RO system or can I use pH Down (using Canna Coco)?

you can use any acid to ph-balance it. don't overpay for "marijuana" branded ph-down... just buy some acetic acid - 5% white distilled vinegar - might need 2-3mL per gallon? pay 5 usd for 3 gallons, not 10-20 dollars for 16 fl oz with branding. those sellers are just raping consumers that don't know any better. .5EC is not so bad. You do not need an RO system. tap coming out at 8ish isn't unusual. It's better for the pipes if slightly alkaline. I have "harder" water than .5EC and don't have issues. if you are in a soilless medium, any fertilizer worth its weight will be ph-buffered and balanced -- meaning even using 8ph out of the tap it should come out to the right range of pH without any adjustment and it will be resistant to ph-drift (buffering part). you really shouldn't have to worry about it. if you do, i'd suggest a better fertilizer that puts a bit more effort into their soilless/hydro products. It's a headache that only comes with substandard soilless/hydro products.