The tank is a 20 gallon tank( yes I know the tank is small. They are stunted due to my lps ) Lighting is 18 watts. Every time I start the light they swim like frantic and hide in a cave.
The light tubes are 2,1 is a white light the other a light blue light. Whys does this happen ?
You might want to consider a dimmer. I know my fish go a bit crazy when I turn on the lights and I think it's just the natural reaction of startling someone. If you were sitting or sleeping in a dark room and someone turns on the lights your reaction would be sit up or get up.
I would suggest that you turn on the room lights first before the tank lights come on. I even have dimmer switched on my room lights that are directly above my 265g tank. Silver Dollars do seem to prefer subdued lighting but sometimes it can't be helped. After all, we do want to see our fish!
Silver dollars are just scared of everything. Speed and numbers are there only defenses. Be careful with them in such a small tank they can get hurt smashing into the glass. They get going so fast and slam into the glass sometimes. How many do you have? They like schools of at least 3 but 6 is better. They don't freak out as much if there are a bunch of them.
I hadn't thought about it in ages, but when I was a kid, I had a tank of silver dollars with no lid and thus no lights on top. It sat next to a window and got indirect sunlight 11-12 hours a day in the sub tropics. The fish were completely mellow, and I had no idea that they were ever scared of anything. They must have liked the fact that they were hard to see!