Valve for CO2 and goldfish

I'd like to include a solinoid valve to shut off the flow of CO2 from a 20
lb tank when the lights are not on. Questions: 

1. Do you put the valve between the tank and the 2-stage regulator or do
you put it after the regulator & needle valve? If you have it after the
regulator, won't the pressure slowly build up to a point that exceeds the
threshold for the solinoid valve?

2. What is a good source for purchasing solinoid valves? I'd appreciate
specific model recommendations.

A comment about goldfish. Someone said that goldfish go dormant (won't
eat) below 55 F and are stressed above 75 F. That is not always the
case. I have goldfish outdoors in 30 gal tanks now. Had them before in
above ground plastic pools also. They are fine in freezing weather; have
to break through the ice though to feed them (pellets). They are slower
and do eat less. Main concern is ice getting thick and expanding - might
crack glass tanks. The tanks are in full sun during the summer and the
water gets above 95 F. They eat and excrete quite a bit - the water gets
to be like pea soup from the algae.

Shiao Y. Wang  
University of Southern Mississippi
sywang at whale_st.usm.edu