RO vs Tap Water For Cannabis

Tap water isn't bad. But it's inconsistent — EC varies by season, alkalinity fights your pH adjustments, and chlorine can affect the biology you're trying to build. RO removes all of that and gives you a clean starting point every time.

The Difference

Tap water contains dissolved minerals, carbonates, bicarbonates, chlorine, and sometimes chloramine — all added or present from the source. The mineral content varies by location and season. EC can range from 0.1 to 0.8 depending on where you live.

Reverse osmosis water pushes tap through a membrane that removes 95-99% of dissolved solids. What comes out is essentially pure water with an EC near 0 and no alkalinity. You build the nutrient profile from scratch, every time, exactly how you want it.

Tap Water

Tap works fine for outdoor living soil grows where the biology buffers fluctuations and you're not dialing in precise nutrient ratios. Many outdoor growers never touch RO and produce excellent results.

For indoor coco in AutoPots, tap water creates two specific problems. First, the alkalinity — carbonates and bicarbonates that resist pH adjustment and cause bounce-back. Second, the inconsistency — you're starting from a different baseline every time you fill the reservoir, which makes dialing in EC harder.

RO Water

The main advantage of RO is consistency. Every reservoir fill starts from the same baseline — zero EC, no alkalinity, neutral-ish pH. You add exactly what you want and nothing else.

For coco in AutoPots this matters because you're building a precise nutrient solution that the plant draws from continuously. Tap water variables compound over time in a bottom-fed system — RO eliminates them.

One thing to remember with RO — because it strips everything, you need to add cal-mag back. Pure RO water with no cal-mag will cause calcium deficiency fast in coco. Always add cal-mag first before other nutrients when mixing RO solution.

Hawaii Water Quality

Hawaii municipal water quality varies significantly by island and neighborhood. Honolulu Board of Water Supply publishes annual water quality reports — worth reading for your specific area.

Generally Hawaii tap EC runs 0.3-0.5 which is moderate. The bigger issue is pH — Hawaii tap often comes out 7.2-7.8 which requires significant pH down to reach 5.9, and the alkalinity causes bounce-back within 24 hours in the reservoir.

My Recommendation

Use RO for indoor AutoPot grows. The investment is $30-50 for a basic under-sink filter and it solves pH instability, EC inconsistency, and chlorine in one shot. Outdoor living soil — tap is fine, the biology handles it.