I feel the same Mel, thats why I water my yard and plants with my tank water. With as large as your tanks are I am sure you could water a whole front/back yard unless your just on acres and acres of land. Could also water a garden if you have one.
Thats the best idea yet. You don't have any pictures of this set up by chance do you Oddball? Thats innovative, must be some sweet setup.My hatchery's roof is epoxy coated metal. I collect rainwater from the 4 gutter drain spouts that drain into poly barrels. The water gravity drains into a primary 2k gal collection tank where a level sensitive sump pump pumps the water through a 5 stage filtration station and into a 5k gallon tank. There's a float switch at the top of the filtered water tank that prevents the sump pump from engaging and overflowing the system. A pond air stone aerates the water off of the hatchery's main blower. Excess rain overflows through 1" pvc, at the tops of the barrels, leading to the back of my property at the drain field. The hatchery is heated instead of individual tanks so the storage tank can heat properly before water changes. The excess water in the barrels also waters my vegetable garden through leading a standard hose (for siphon) into a drip hose line.

Nitrates are only a small part of the picture. The DOC's need to be removed to.
If you want to change less water, keep fewer fish.