A solenoid is just what you said, an electronically controlled on/off switch that can be used in conjunction with a pH controller or a timer. I'll forego the solenoid and run the CO2 all the time until I can save up for a pH controller.
A bubble counter is mostly necessary in certain reactors where you can't detect the CO2 flowrate otherwise. If you're just putting an airline underneath a powerhead, or any other method where the airline is submerged and you can count bubbles, then you won't need a bubble counter, either.
Search Ebay for CO2 regulator and you'll find some killer deals. That, and a needle valve, and you're set!
- Sam P -
plantedtanker in limbo - all tanks currently in storage