Do I really need to buy a reverse osmosis filter or can I just use tap water? My tap water PPM is around 180. Will this cause problems for nutrient ratios or is it fine for most plants?

180 PPM is in the gray zone - not ideal but definitely workable with some adjustments. Here's the honest breakdown.

Why PPM number alone isn't enough

PPM tells you the total dissolved solids in your water but not what those solids are. 180 PPM of calcium carbonate (hard water) behaves very differently from 180 PPM of sodium chloride (saline) or 180 PPM of bicarbonate (alkaline buffering).

Before making a decision, request your municipal water quality report (called a Consumer Confidence Report in the US - your water utility must provide one annually). The key numbers to look at:

• Calcium (Ca) - common in hard water; usually helpful to plants but can cause tip burn in lettuce at very high levels
• Magnesium (Mg) - generally beneficial; hard to have too much
• Sodium (Na) - problematic above 50 mg/L; plants cannot use it and it accumulates
• Chloride (Cl) - problematic above 100 mg/L; damages sensitive crops like lettuce
• Bicarbonate (HCO3⁻/KH) - high alkalinity creates pH drift problems (see our pH drift Q&A)

At 180 PPM: the practical answer

Likely fine for: basil, mint, chives, spinach, kale - all are moderately hard-water tolerant.

May cause issues with: lettuce (particularly tip burn-prone varieties), cilantro, and strawberries - crops that are sensitive to sodium, chloride, or calcium excess.

What you need to adjust:

• Start your nutrient solution knowing you already have ~90–120 mg/L of minerals present. Reduce or eliminate Cal-Mag supplementation - you're likely already getting adequate calcium and magnesium from your tap water.
• Mix nutrients to EC 1.0–1.2 instead of 1.4 initially - your 180 PPM baseline (≈0.36 mS/cm) is already contributing to total EC.
• Track EC over a reservoir cycle (1–2 weeks). If EC rises without added nutrients, your water's mineral content is accumulating - dilute with plain water or do a partial reservoir flush.

When to invest in RO

Consider RO if any of these apply:

• Your water report shows sodium above 50 mg/L or chloride above 100 mg/L
• You're trying to grow sensitive crops (lettuce, strawberries, watercress) and seeing consistent leaf edge issues
• Your reservoir EC climbs unpredictably between reservoir changes
• You want precise nutrient control and prefer to build your formula from a blank slate

A basic under-sink RO system produces water at 5–20 PPM and costs $150–$250. It's a one-time investment that simplifies nutrient management significantly if you grow year-round.

The cheap intermediate option

Many growers blend 50/50 tap and RO water. This halves the starting PPM to ~90 and reduces sodium and chloride to safe levels while costing less than full RO for all water.

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This answer is reviewed against published horticultural research. It is not a substitute for advice from a certified horticulturalist.