If you prefer to cowash, here are a couple of ‘clarifying’ conditioners that may work. They have EDTA which is what’s needed to remove hard water buildup (and any product buildup). You can also try searching for “clarifying cowash conditioners” and “cleansing low poos” for other suggestions.
VO5 Vanilla Mint Tea Clarifying Conditioner
Water, Cetyl Alcohol, Stearalkonium Chloride, Cetrimonium Chloride, Glyceryl Stearate, DMDM Hydantoin, Disodium EDTA, Polysorbate 20, Propylene Glycol, Stearyl Alcohol, Steareth 21, Vanilla Planifolia Fruit Extract, Tocopheryl Acetate, Panthenol, Ascorbic Acid, Mentha Piperita (Peppermint) Leaf Extract, Camellia Sinensis Leaf Extract, Niacinamide, Biotin, Fragrance, Butylphenyl Methylpropional
Suave Daily Clarifying Conditioner
Water (Aqua), Cetyl Alcohol, Cetrimonium Chloride, Potassium Chloride, Fragrance (Parfum), Distearyldimonium Chloride, Disodium EDTA, 2-Bromo-2-Nitropropane-1, 3-Diol, Pvp, Methylchloroisothiazolinone, Methylisothiazolinone, Tocopheryl Acetate (Vitamin E Acetate)
Here are some articles (mostly by Tonya McKay) that may also interest you:
Use a Clarifying Shampoo to Remove Conditioner Build-Up
CurlySuzy Q&A: Looking for Products that Contain EDTA
Is Water Damaging My Hair?
What Makes a Cleansing Ingredient More Harsh or More Gentle?
Is Hard Water Ruining Your Curls?