hello and welcome!
have you looked around the main portion of this site? it gives very detailed and and only the most important info. check it out! this site really aids me and my turtle keeping.
first off, how big is the turtle? you mentioned that your uncle has had it for 20+ years... thats amazing! especially after hearing what kind of condition hes been in. can you return the 30 gallon by any chances? if your turtle is any bigger than 3 inches, it is going to need a lot - A LOT - more room.
the not-as-bright and purplish light is a fluorescent light. did it come with the 30 gallon set up? you need to provide 2 things for your turtle when it comes to lighting: 1) UVB rays and 2) heat.
fluorescent lights do not emit much heat but can be provided by a regular house hold bulb, like the one your uncle had. the light you have now probably does not provide sufficient UVB. you're gonna have to look for a SPECIALLY designed bulb (available only @ pet stores and online) that is designed for such purpose.
here is a very readable guide to turtle lighting:
http://www.redearslider.com/lighting.html
and a link that tells you exactly what kind of bulbs you'll be looking for:
http://www.redearslider.com/light_sources.html