In Understanding Tank Water Quality I explained that the typical tank outlets is located close to the bottom of the tank. This takes the dirtiest water from the bottom of the tank,rather than the cleanest water near the top.
So what can you do?
Well the simplest solution is to put a second outlet further up the tank, as shown below.
The top outlet is then used for the supply to the house, while the lower outlet is used to supply a garden tap.
The following schematic shows a typical installation.
Normal operation is with Valve 1 open supplying the pump, and Valve 2 open supplying a garden tap. Valve 3 is left shut.
The garden top would only have the pressure of the tank so it could only be used for low pressure jobs like filling a watering can. At extra cost you could add a separate pump.
When the tank level drops below the top outlet Valve 3 can be opened to supply the pump. Although this water is taken from the bottom of the tank most of the substandard water should have been drained off to the garden.
The outlet should also be on the opposite side of the tank to the Inlet.
Outlet Modification shows a low cost modification to further improve the quality
To help you pick a tank see Round or Slimline Tanks