Yeah - the cooling system capacity of my '95 Integra GS-R was less than 2 gallons. The plastic radiator tank finally cracked at
130K miles, but I understand this isn't unusual with plastic tanks.I always mix with distilled water. However - I did a few flushes with out local tap water in the Berkeley, California area, which is very soft (checked with an aquarium water test kit). I lived in Santa Clara, where 2/3 of the water comes from wells in the city. The water leaves hard water stains everywhere. A measurement showed it was around 250 ppm of calcium/magnesium, or about 8 times the level of the water around Berkeley. I preferred not to wash my car with that water.
I once bought a bottle of Honda coolant mail-order (for $7) along with oil filters. However - typical dealer prices were in the $10-13 range. I just bought a bottle of Prestone Extended-Life for $9 at a local parts store. It's about $2 less at Wal-Mart, but I didn't want to make the trip.
Seriously - Honda doesn't care about phosphates, but goes bonkers over silicates. In addition to the water pump issues, don't silicates eventually gel out? Those tiny passages could get clogged awfully quickly.