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i got 5.0 UVB light for my two female a little smaller than medium sized RES. they havent basked in it once and hide underneath the anacharis plants i have floating in the water

just got the UVB light today. they are two rescued turtles that ive only have for about 3 days
in a cpl hours i will try using it with the heat lamp on at the same time.

am i just over thinking this?

It's possible. A 5.0 should be fine, though you might have to do a burn in with it first. Do you know the basking area temps?

You should have no worries about a 5.0 being too strong. If the fixture has an aluminum or shiny reflective backing, it will intensify the strength somewhat. What type of UVB light did you get---a compact flourescent or a tube. If it's a Reptisun 5.0 tube, you don't need a burn-in period. This type of bulb is fine 6 - 8 inches from the basking area. Basking area temp should be in the range of 88 - 90F.