redearslider.com

I currently use a mercury vapor lamp for my RES. However, it has to be at least 18 inches away, and when I keep it that far away my basking area doesn't get warm enough. I've heard basking temps should be 90-95 degrees, or that they just need to be 10 degrees warmer at least than the water temperature (which is currently about 70). Which is correct?
Also, is it okay for my water to be cooler in the winter? it varies from 70-72 right now, 75ish in the summer. Basking right now varies from 75-82 (it's usually higher but we literally don't have heat in my house right now, I'm heating my bedroom with a space heater).

What are you using to measure the temps? I would say 10 degrees is the minimum necessary to entice them, but I think my RES like it a little warmer.

What wattage bulb do you have? If you have an older turtle, 72 should be fine.

I have a 100 watt mega ray. I also keep my water around 75* and basking area 10* above that (85* ) for a five year old slider. But I live in an old house that just gets cold so I have a halogen 50 watt bulb on a dimmer to supplement it by one or three degrees or as needed. The dimmer allows me to fine tune it very well.
Most of the time I do not need the extra heat but it's there for the ready.
https://www.lightingdirect.com/lutron-t ... onttcl100h