The inside walls should be smooth and clean with no build up. Cleaning the cylinder is a must if foreign material is seen. Rust mounds may be acting as a cork, and if later jarred loose could expose a pin hole leak through the cylinder wall when filled. Always inspect again after cleaning for cleanliness or structural damage. Care must be taken not to drop the light or bang it into the bottom of cylinders which are to short for the light.
Every light is designed for specific size tanks.
This will extend the life of the bulbs and cut down the damaging heat build-up in the tube. When you disconnect a light from the power supply disconnect it by the connectors not by pulling on the wires. This will prevent the connectors from unscrewing. Don't change the shape of the handle, this may damage the power cord or break off the handle. We have drop lights that will meet that need.