Yes
grow rocks can have a build up of fertilizing salts over time.
I would flush the system when you change the
nutrient solution. Every few weeks. An easy way to do that is fill your tank up with clean water and let it water a few dozen times. Or if you have access to a drain then unhook the
pump and run the hose from the garden to the drain. Then use a water hose and run clean water from the top of each plant cup. That will flush all the salts from the rocks and out though the system to the drain.
Yes that is exactly how you do it.
That is what I like about grow rocks so much is that they hold so much oxygen between watering. One of the reasons why hydroponic growing is so much faster is because you are force feeding the plants by watering them if they want it or not. As long as your pH and PPM is correct for the plants age then you can't water to much. Most plants do need a dry out period so don't think by watering all the time will increase yields.
Conversion
bulbs are OK if you are on a budget but if you can afford it I would go with the 1000
hps because theirs is no such thing as to much light when it comes to increased yield. Especially in the bud room.