R/O water should have a pH around 5.5 or so. So adding 5.5 pH R/O should "dilute" your 7.8 pH water, thus lowering it.
Otherwise you can use other acidic sources via active substrates (acidic buffering substrates such as ADA Aqua soil), tannin/humic acid sources like peat moss, Indian Almond Leaves, Alder Cones, Cholla wood, driftwood tannins, etc. I think there are tannic/humic acid tinctures (liquid), and I think I've heard of them in concentrated powder forms as well, though research would need to be done to know how to go about properly/safely using them.
EDIT: Oh I missed the part about it being a shrimp tank. I would assume this is for soft acidic Caridinas, in that case I would definitely recommend a active acidic buffering substrate. It keeps the water stable in the acidic range, which both (acidic water and stability) are crucial to keep Caridinas healthy. I am not all too familiar with what substrates are best for that, but ADA aqua soils are common. Browse them. You can ask for recommendations on The Shrimp Spot forum (tons of experienced help there) and/or do searches for best Caridina shrimp substrates.
With a KH of 0-1 though, and using R/O water, your pH should be a lot lower.
What's your GH?
If you have to, (actually it is really recommended when dealing with shrimp, especially Caridinas), use 100% R/O water and just remineralize.
If you are already, what remineralizer product(s) are you using?