Hello and welcome
Placing the UVB over water will not be sufficient. It produces regular light and UVB rays , what you see is the white light regular rays passing through the water to the bottom. The UVB rays can only pass a few inches into the water. Hope you still have the box and or the instructions that came in the box. UVB placement is very important and must be set directly above basking area , they will not penetrate glass nor most plastic's and screen cover's block a lot to. Hopefully a good brand will also give a chart for white fixtures and polished aluminum fixtures , that makes a difference also. Should go directly above the wire cage top on the Penn Plax turtle topper at the proper distance your brand of UVB states..
Here's only an "example" of a popular one , may not be yours , only to give you an idea :
http://www.redearslider.com/downloads/r ... ctions.pdf