[simonlucas98]

Right now, I've got a 6.5 gallon half-hex tank. The inhabitants: Dart the blue veiltail, five neon tetras, and one ghost shrimp, Ichabod. After repeated fin rot attacks on Dart, which all healed, and ammonia levels not decreasing, stuck at 2.0, I am going to move them to a 20 gallon. It was previously used as the home of two Jack Dempsey cichlids, but has sat in my garage since 2004-ish when the last one died. I have a new filter, a Tetra EX20, and a reliable heater, thermometer, everything. However, the old 20 gallon still has all its hood parts, including the light. It is a fluorescent actinic bulb, I am pretty sure, and is still in working order. Will this bulb be appropriate for my tank? I am interested in having real plants, and have one right now, a rosette sword. Will the bulb affect the colors of my present fish? Will any plants I have do well under the light? Thank you.

 

[Pasfur]

However, the old 20 gallon still has all its hood parts, including the light. It is a fluorescent actinic bulb, I am pretty sure, and is still in working order. Will this bulb be appropriate for my tank?

An actinic bulb would shine with a deep blue color. It is certainly not what you want. If you are really interested in this being a live plant tank, then I would start by reading this sticky, which articulates the details you will need far better than I am capable:
http://www.tropicalfishkeeping.com/...sic-approach-natural-planted-aquarium-355178/